(CC.     o-f        ).    K    reao^nt    -   .OO  | 
f~  o  u  i  /  a  I  a-  n  "t"  . 


QUALITATIVE   CHEMICAL   ANALYSIS 


ARTHUR  A.   NOYES 


THE  MACMILLAN  COMPANY 

NBW  YORK  •    BOSTON   •    CHICAGO  •   DALLAS 
ATLANTA  •    SAN   FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON  •    BOMBAY  •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 
TORONTO 


A  COURSE   OF   INSTRUCTION 

IN  THE 

QUALITATIVE 
CHEMICAL   ANALYSIS 

OF  INORGANIC  SUBSTANCES 


BY 
ARTHUR  A.  NOYES 

DIRECTOR   OF   CHEMICAL   RESEARCH 
CALIFORNIA   INSTITUTE  OF   TECHNOLOGY 


EIGHTH  EDITION,  ENTIRELY  REWRITTEN 


NEW  YORK 

THE   MACMILLAN   COMPANY 

LONDON:    MACMILLAN  &  CO.,  LTD. 

1920 


COPYRIGHT,  1897  AND  igso, 
BY  ARTHUR  A.  NOYES. 


Eighth  edition  set  up  and  electrotyped. 
Published  September,  1920. 


XortoooO  iprcss 

J.  8.  Gushing  Co.  —  Berwick  <fe  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


EMS 
Lib. 


PREFACE 

THIS  text-book  is  an  attempt,  on  the  experimental  side,  to  train  the 
student  of  qualitative  analysis  in  careful  manipulation  and  exact  methods 
of  procedure,  such  as  are  commonly  employed  in  quantitative  analysis. 
It  is  an  attempt,  on  the  theoretical  side,  to  make  clear  to  the  student  the 
reason  for  each  operation  and  result,  and  to  accustom  him  to  apply  to  them 
the  laws  of  chemical  equilibrium,  and  especially  the  principles  relating  to 
solubility  and  to  the  ionization,  complex-formation,  and  oxidation  and  re- 
duction of  substances  in  solution.  It  is  believed  that  in  both  these  ways 
the  educational  value  of  the  subject  is  greatly  increased. 

The  book  is  divided  into  two  main  Parts,  entitled  The  Course  of  In- 
struction and  The  System  of  Analysis.  In  presenting  the  System  of  Analysis 
the  description  of  the  operations  is  separated  sharply  from  the  discussion 
and  explanation  of  them.  The  operations  are  described  with  as  great 
definiteness  as  possible  in  short  paragraphs  entitled  Procedures ;  and  each 
of  these  is  followed  by  Notes  in  which  are  given  the  reasons  for  the  opera- 
tions, the  precautions  necessary  and  difficulties  encountered  in  special 
cases,  the  chemical  behavior  of  the  different  constituents,  the  indications 
afforded  of  their  presence,  and  the  application  of  the  theoretical  principles 
to  the  reactions  involved. 

This  System  of  Analysis  is  the  result  of  many  years'  researches,  during 
which  the  goal  striven  for  has  been  gradually  approached  in  the  way  illus- 
trated by  the  successive  editions  of  this  book.  This  goal,  by  no  means 
yet  fully  attained,  has  been  the  development  of  the  simplest  possible  methods 
that  will  provide  for  the  reliable  detection  of  a  small  quantity  (i  mg.)  of 
any  constituent  in  the  presence  of  a  large  quantity  (500  mg.)  of  any  other 
constituent.  The  effort  has  also  been  made  to  avoid  the  use  of  tests,  such 
as  oversensitive  color  reactions,  flame  colorations,  and  bead  tests,  that  do 
not  enable  the  amounts  of  the  various  constituents  present  to  be  approxi- 
mately estimated;  for  a  satisfactory  scheme  of  qualitative  analysis  care- 
fully executed  can  be  made  to  furnish  this  important  information,  thus 
often  making  unnecessary  a  more  laborious  quantitative  analysis. 

In  the  researches  by  which  the  System  of  Analysis  has  been  brought  into 
its  present  form,  the  author  has  had  the  able  cooperation  of  many  of  his 


vi  PREFACE 

associates  at  the  Massachusetts  Institute  of  Technology  and  the  California 
Institute  of  Technology.  To  Professors  W.  C.  Bray  and  E.  B.  Spear  belongs 
in  largest  measure  the  credit  for  the  method  of  analysis  of  the  aluminum 
and  iron  groups,  to  Professor  W.  C.  Bray  that  for  the  alkaline-earth  group, 
and  to  Professor  Graham  Edgar  that  for  the  detection  of  acidic  constituents 
in  non-igneous  products  as  well  as  for  improvements  in  many  of  the  pro- 
cedures of  the  copper  and  tin  groups.  The  assistance  and  advice  of  Mr. 
Ernest  H.  Swift  has  been  of  great  value  in  the  final  revision  of  the  whole 
scheme  of  analysis.  The  author  has  also  received  many  important  sugges- 
tions from  Professors  Henry  Fay,  W.  T.  Hall,  A.  A.  Blanchard,  Edward 
Mueller,  L.  F.  Hamilton,  H.  J.  Lucas,  and  Mr.  Roger  Williams. 

The  Course  of  Instruction  includes  two  sections  —  one  entitled  Labora- 
tory Experiments,  giving  the  directions  for  the  laboratory  work;  and  the 
other  entitled  Questions  on  the  Experiments,  consisting  of  a  series  of  ques- 
tions to  be  studied  in  connection  with  the  class-room  exercises. 

The  laboratory  work  described  in  the  section  on  Laboratory  Experiments 
is  from  beginning  to  end  closely  correlated  with  the  systematic  scheme  of 
analysis.  For  experience  has  convinced  the  author  that  the  plan  followed 
in  many  text-books  of  requiring  the  student  to  study  the  separate  reactions 
characteristic  of  the  various  elements  before  undertaking  their  systematic 
separation  is  highly  unsatisfactory.  However  valuable  the  knowledge  of 
the  additional  reactions  might  be,  it  is  found  in  practice  that  the  perform- 
ance of  so  large  a  number  of  independent,  disconnected  experiments  makes 
little  impression  on  the  student's  mind  and  fails  to  awaken  his  interest  hi 
the  subject.  Qualitative  analysis  affords  an  effective  means  of  teaching  a 
part  of  inorganic  chemistry  chiefly  because  it  unites  into  a  connected  whole 
a  great  variety  of  isolated  facts,  and  because  the  student  sees  a  practical 
use  of  the  information  presented  to  him;  but  these  advantages  evidently 
do  not  apply  to  facts  not  directly  related  to  the  process  of  analysis. 

The  Questions  on  the  Experiments  do  not  in  general  include  such  purely 
informational  questions  as  are  immediately  suggested  by  the  Notes  on  the 
Procedures.  They  are  mainly  intended  to  assist  the  instructor  in  training 
his  students  more  fully  in  the  general  principles  involved  and  in  enabling 
them  to  derive  from  the  subject  the  mental  training  it  is  capable  of  afford- 
ing. They  are  in  large  part  of  such  a  character  that,  in  order  to  answer 
them  properly,  the  student  must  not  only  carefully  study  the  Notes  on  the 
Procedures,  but  must  also  do  independent  thinking.  It  is  assumed  in  these 
questions,  as  well  as  in  the  Notes  on  the  Procedures,  that  the  student  has 
previously  acquired,  in  his  course  on  Inorganic  Chemistry,  a  general  knowl- 
edge of  the  mass-action  law  and  of  the  chemical  aspects  of  the  ionic  theory. 
To  what  extent  the  instructor  will  make  use  of  the  Questions  will  depend 
on  the  time  available  for  the  course  and  on  the  maturity  of  his  students. 


PREFACE  vii 

To  make  the  course  fully  effective  from  an  educational  standpoint,  it 
must  be  so  conducted  as  to  overcome  the  tendency  of  students  to  rush  the 
laboratory  experiments  and  to  carry  out  the  Procedures  in  a  routine,  un- 
intelligent way.  To  this  end  the  laboratory  work  must  be  supplemented 
by  many  class-room  conferences  with  small  sections  of  15  to  25  students; 
thus  there  should  be  one  such  conference  preceding  every  two  laboratory 
exercises,  or  in  the  early  stages  of  the  course  preceding  each  laboratory 
exercise,  which  should  if  possible  be  three  hours  long.  The  laboratory  and 
class-room  exercises  should,  moreover,  be  so  correlated  as  to  induct  the 
student  rather  gradually  into  the  detailed  scheme  of  analysis  of  each  group, 
but  finally  to  secure  by  frequent  repetition  his  full  understanding  of  it. 
The  best  plan  of  doing  this,  in  the  author's  opinion,  involves  four  steps  as 
follows :  (i)  Before  beginning  the  laboratory  work  on  any  group  the 
students  are  required  to  learn  the  outline  of  the  process  and  the  chemical 
reactions  upon  which  it  is  mainly  based,  by  studying  the  Table  summarizing 
the  analysis  of  that  group  and  by  reciting  upon  it  in  the  class-room;  but 
they  should  not  be  asked  to  learn  in  advance  the  details  of  the  Procedures 
nor  the  contents  of  the  Notes  upon  them.  (2)  The  students  then  work 
through  in  the  laboratory  the  Procedures  of  the  group  with  a  known  solu- 
tion (as  described  in  the  Laboratory  Experiments),  referring  to  the  Notes, 
especially  those  with  an  experimental  bearing.  (3)  They  are  then  required 
to  study  the  Notes  more  carefully,  including  those  describing  the  principles 
involved,  and  (unless  the  course  is  a  brief  elementary  one)  to  answer  in 
writing,  or  prepare  themselves  to  answer  in  the  class-room,  the  correspond- 
ing Questions  on  the  Experiments ;  all  these  matters  being  then  taken  up 
very  fully  in  the  class  conferences,  with  the  help  of  written  tests,  oral  ques- 
tioning, and  explanations  by  the  instructor.  (4)  After  this  full  discussion 
the  students  review  the  group  by  analyzing  in  the  laboratory  one  or  more 
unknown  solutions,  as  directed  in  the  Laboratory  Experiments.  —  In  carrying 
out  this  plan  it  is  desirable  to  keep  the  members  of  the  class  nearly  together 
in  their  laboratory  work,  which  may  be  accomplished  by  giving  to  the  faster 
working  students  additional  unknown  solutions  on  each  group,  and  by  allow- 
ing those  who  are  falling  behind  to  omit  some  of  the  less  important  experi- 
ments, or  to  work  overtime.  In  the  laboratory  great  stress  is  laid  on  careful 
work,  such  as  will  enable  the  proportions  of  the  various  constituents  present 
in  unknown  solutions  to  be  estimated  and  small  quantities  of  them  to  be  de- 
tected. An  effective  means  of  teaching  the  details  of  manipulation,  es- 
pecially when  the  classes  are  large,  is  for  the  instructor  to  carry  through 
in  the  lecture-room,  after  the  students  have  had  a  little  experience  of  their 
own  in  the  laboratory,  the  complete  process  for  the  analysis  of  the  copper- 
group. 


viii  PREFACE 

Even  when  the  time  available  for  the  subject  of  qualitative  analysis  does, 
not  permit  of  so  complete  a  course  as  that  here  presented,  the  student  gets, 
in  the  author's  opinion,  a  better  training  by  working  through  selected  parts 
of  an  exact  scheme  of  analysis  carefully  and  thoroughly  than  he  does  by 
covering  the  whole  of  an  elementary  scheme  superficially.  Experiments 
that  may  be  well  omitted  in  briefer  courses  are  indicated  by  asterisks  pre- 
fixed to  the  description  of  them  in  the  section  entitled  Laboratory  Experi- 
ments. 

Pasadena,  California,  Augusi,  1920. 


CONTENTS 


PART  I.    THE  COURSE  OF  INSTRUCTION. 

PAGE 

LABORATORY  EXPERIMENTS i 

QUESTIONS  ON  THE  EXPERIMENTS 19 


PART  II.     THE   SYSTEM  OF  ANALYSIS. 

PREPARATION  OF  THE  SOLUTION  FOR  THE  DETECTION  OF  THE  BASIC 
CONSTITUENTS 

General  Directions 39 

Preparation  of  the  Solution 42 

DETECTION  OF  THE  BASIC  CONSTITUENTS 

General  Discussion 58 

Separation  into  Groups 60 

Precipitation  and  Analysis  of  the  Silver-Group  61 

Precipitation  and  Separation  of  the  Copper  and  Tin  Groups        .  66 

Analysis  of  the  Copper-Group       .......  76 

Analysis  of  the  Tin-Group    ........  82 

Detection  of  Phosphate         ........  89 

Precipitation  and  Separation  of  the  Aluminum  and  Iron  Groups  .  90 

Analysis  of  the  Aluminum-Group          ......  98 

Analysis  of  the  Iron-Group  ........  102 

Precipitation  and  Analysis  of  the  Alkaline-Earth  Group       .        .113 

Analysis  of  the  Alkali-Group :   Shorter  Less  Exact  Method  .         .  121 
Analysis  of  the  Alkali-Group :  Exact  Method      .        .        .        .125 

Supplementary  Procedures  for  Basic  Constituents        .        .        .129 


x  CONTENTS 

PAGE 

DETECTION  OF  THE  ACIDIC  CONSTITUENTS 

General  Discussion 133 

General  Directions 135 

ANALYSIS   OF  NON-IGNEOUS  PRODUCTS 

Preparation  of  the  Solution  and  Directions  for  Its  Treatment      .  136 

Behavior  of  the  Acidic  Constituents  toward  Group-Reagents       .  140 

Analysis  of  the  Chloride-Group 144 

Analysis  of  the  Sulfate-Group 155 

Detection  of   Other   Constituents    in   the   Sodium   Carbonate 

Solution 158 

Detection  of  Carbonate  and  Sulfide  in  the  Original  Substance      .  163 

ANALYSIS   OF  NATURAL  SUBSTANCES  AND  IGNEOUS  PRODUCTS 

Detection  of  Sulfate,  Carbonate,  Sulfide,  and  Cyanide         .        .166 
Detection  of  Chloride,  Fluoride,  and  Borate       .        .        .        .167 


APPENDIX. 

I.    PREPARATION  OF  THE  REAGENTS 171 

II.    PREPARATION  OF  THE  TEST-SOLUTIONS 175 

III.  APPARATUS  REQUIRED 177 

IV.  SOLUBILITIES 178 

V.    IONIZATION  VALUES 180 

VI.    SPECIFIC  REDUCTION-POTENTIALS 181 

VII.  ATOMIC  WEIGHTS  OF  THE  COMMON  ELEMENTS        .        .        .182 


CONTENTS  xi 

TABLES  OUTLINING  THE  SYSTEM   OF  ANALYSIS. 


I.  Preparation  of  the  Solution  for  the  Detection  of  the  Basic 

Constituents 42 

II.  Separation  of  the  Basic  Constituents  into  Groups         .        .  60 

III.  Analysis  of  the  Silver-Group 61 

IV.  Precipitation  and  Separation  of  the  Copper  and  Tin  Groups  66 
V.    Analysis  of  the  Copper-Group 76 

VI.    Analysis  of  the  Tin-Group 82 

VII.  Precipitation  and  Separation  of  the  Aluminum  and  Iron 

Groups 90 

VIII.     Analysis  of  the  Aluminum- Group 98 

IX.     Analysis  of  the  Iron-Group 102 

X.  Separation  of  Zinc,  Cobalt,  and  Nickel         ....  108 

XI.     Analysis  of  the  Alkaline-Earth  Group 113 

XII.  Analysis  of  the  Alkali-Group :  Shorter  Less  Exact  Method  .  121 

XIII.  Analysis  of  the  Alkali-Group :  Exact  Method       .        .        .125 

XIV.  Supplementary  Procedures  for  Basic  Constituents        .        .  130 
XV.  Detection  of  Groups  of  Acidic  Constituents  ....  140 

XVI.  Separation  of  the  Chloride-Group  into  Subgroups         .        .  144 

XVII.     Detection  of  the  Separate  Cyanides 148 

XVIII.  Detection  of  Thiocyanate,  Iodide,  Bromide,  and  Chloride    .  150 

XIX.  Detection  of  Sulf  ate,  Sulfite,  Chromate,  Fluoride ,  and  Oxalate  155 

XX.  Detection  of  Nitrate,  Nitrite,  Borate,  Arsenate,  and  Arsenite  158 

XXI.  Detection  of  Sulfate,  Carbonate,  Sulfide,  and  Cyanide  in 

Natural  and  Igneous  Substances    .        .        .        .        .166 

XXII.  Detection  of  Chloride,  Fluoride,  and  Borate  in  Natural  and 

Igneous  Substances 167 


PART    I 
THE    COURSE    OF    INSTRUCTION 


LABORATORY  EXPERIMENTS 


GENERAL  DIRECTIONS 

Preliminary  Work.  —  Check  off  on  an  apparatus  list  (corre- 
sponding to  that  printed  in  the  Appendix)  the  apparatus  found  in 
the  desk,  and  sign  and  hand  in  the  list. 

Make  a  750  cc.  wash-bottle,  taking  pains  to  bend  the  tubes 
and  to  cut  them  off  so  as  to  correspond  closely  with  the  model 
exhibited  in  the  laboratory.  Make  also  a  250  cc.  wash-bottle 
(for  washing  with  hot  water  and  special  solutions). 

Make  a  dropper  about  10  cm.  (4  inches)  long  by  drawing  out 
one  end  of  a  glass  tube  to  a  fairly  wide  capillary  and  slightly 
expanding  the  other  end  with  the  aid  of  a  file  while  it  is  heated 
in  a  flame.  Cap  the  expanded  end  with  a  rubber  nipple.  De- 
termine how  many  drops  the  dropper  delivers  to  make  i  cc. ; 
and,  unless  the  number  is  within  3  or  4  of  30,  widen  or  constrict 
the  orifice  till  this  is  the  case.  Scratch  on  the  dropper  with  a 
file  a  circle  at  the  place  where  the  volume  is  i  cc. 

Make  3  stirring-rods  about  15  cm.  long  by  cutting  a  piece  of 
glass  rod  into  sections  and  then  rounding  the  ends  in  a  flame. 

Directions  for  Study.  —  Before  carrying  out  each  of  the 
following  experiments  study  the  table  (or  other  assignment) 
referred  to  at  the  beginning  of  the  experiment.  While  making 
the  experiment  or  after  completing  it,  read  the  "Notes"  re- 
ferred to  at  the  end  of  the  experiment.  After  completing  it, 
study  the  questions  on  it  contained  in  the  chapter  entitled 
"Questions  on  the  Experiments"  (pages  19-38),  writing  out 
the  answers  to  them  or  being  prepared  to  recite  upon  them  in 
the  classroom,  as  the  instructor  may  direct. 


2  LABORATORY  EXPERIMENTS 

Directions  as  to  the  Note-book. — In  the  case  of  each  of  the 
experiments  record  in  the  note-book  the  operations  very  briefly ; 
but  record  everything  that  happens  fully,  though  concisely. 
Write  equations  expressing  all  the  chemical  changes  that  take 
place.  In  these  equations  represent  solid  substances  by  under- 
lining their  formulas,  denote  largely  ionized  dissolved  substances 
by  attaching  to  their  formulas  -f  and  —  signs  in  such  a  way  as 
to  show  the  ions  into  which  they  dissociate,  and  show  slightly 
ionized  dissolved  substances  by  omitting  these  signs  from  their 
formulas.  Thus  the  report  on  Expt.  i  would  be  made  in  the 
following  form : 

Eipt.  i.  —  Added  HNO3 :  no  change  observed. 
Added  NH4C1 :  white  curdy  ppt. 

Ag+N03-+NH4+Cl- = AgCl+NH4+NO,-. 
Passed  in  H2S :  large  black  flocculent  ppt. 
Cu++(N03-)2-|-H2S=CuS+2  H+NOr. 

In  regard  to  the  solubility  and  ionization  of  substances,  see  the 
corresponding  tables  in  the  Appendix. 


LABORATORY  EXPERIMENTS  3 

DETECTION    OF   THE   BASIC   CONSTITUENTS 

Experiment  i.  —  Separation  of  the  Basic  Constituents  into 
Groups.  —  Read  the  General  Discussion  on  page  58,  and  study 
Table  II  (page  60).  Measure  out  in  a  10  cc.  graduate  5  cc. 
portions  of  the  test-solutions  (see  Note  i)  of  AgNO3,  Cu(NO3)2, 
Zn(NO3)2,  Ca(NO3)2,  and  KNO3.  Mix  the  portions  in  a  conical 
flask,  add  5  cc.  of  6  n.  (6  normal)  HNO3  and  4  cc.  of  3  n.  NH4C1 
solution,  shake  the  mixture  for  a  minute  or  two,  and  filter  it. 
Dilute  the  nitrate  with  water  (see  Note  2)  to  a  volume  of  100  cc. 
Pour  it  into  a  200  cc.  conical  flask ;  insert  a  two-hole  rubber 
stopper  through  which  passes  a  tube  leading  to  the  bottom  of 
the  flask;  and  pass  in  through  a  gas- wash-bottle  a  moderate 
current  of  H2S,  till,  upon  closing  the  open  hole  in  the  stopper 
with  the  finger  or  with  a  piece  of  glass  rod,  the  gas  no  longer 
bubbles  through  the  wash-bottle.  Filter  the  mixture.  To  the 
filtrate  add  10  cc.  of  NH4OH  and  3  cc.  of  6  n.  (NH4)2S  solution. 
Shake  the  mixture  and  filter  it.  Evaporate  the  filtrate  to  a 
volume  of  about  iocc.,  filter,  and  to  the  cold  solution  add  5  cc.  of 
(NH4)2C03  reagent  and  5  cc.  of  ethyl  alcohol. 

Notes.  —  i .  The  solutions  of  constituents  to  be  tested  for,  here 
called  the  test-solutions,  are  all  so  made  up  as  to  contain  10  mg.  (10 
milligrams)  of  the  constituent  per  cubic  centimeter  of  solution.  The 
mixture  used  in  this  experiment  therefore  contains  50  mg.  of  each  of 
the  basic  constituents  silver,  copper,  zinc,  calcium,  and  potassium. 
The  student  should  acquire  the  habit  of  working  with  definite  quanti- 
ties of  the  constituents  and  of  noting  the  size  of  the  precipitates  which 
they  yield.  For  a  good  qualitative  analysis  should  not  only  show  the 
presence  or  absence  of  the  various  constituents,  but  should  also  furnish 
an  estimate  of  the  proportions  in  which  they  are  present. 

Test-solutions  should  not  be  used  in  place  of  reagents,  nor  reagents 
in  place  of  test-solutions,  since  the  concentrations  are,  as  a  rule,  quite 
different.  Unless  otherwise  specified,  all  salt  solutions  used  as  reagents 
are  i  normal,  and  all  acid  or  base  solutions  are  6  normal.  The  sig- 
nificance of  the  term  normal  is  explained  in  Note  4,  P.  n. 

2.  Distilled  water  should  always  be  employed  in  qualitative 
analysis,  and  this  is  to  be  understood  when  water  is  mentioned. 


4  LABORATORY  EXPERIMENTS 

Experiment  2.  —  Precipitation  of  the  Silver-Group.  Principles 
Relating  to  Equivalents,  Concentration,  and  Solubility-Effect.  — 
Study  P.  ii  (Procedure  n  of  the  System  of  Analysis)  and  the 
Notes  on  it. 

Prepare  about  20  cc.  of  a  3  n.  NH4C1  solution,  describing  in 
the  note-book  just  how  it  is  done. 

Pour  into  a  test-tube  just  locc.  of  the  test-solution  of  Pb(NOa)2, 
and  add  from  a  dropper  the  3  n.  NH4C1  solution,  3  drops  at  a 
time,  till  after  shaking  a  precipitate  remains.  Calculate  approxi- 
mately the  normal  concentration  of  the  lead  and  that  of  the 
chloride  in  the  solution  just  before  the  permanent  precipitate 
first  forms,  and  the  corresponding  value  of  the  ion-concentration 
product  for  lead  chloride  (assuming  the  salts  are  completely 
ionized).  Find  the  ratio  of  this  product  to  the  saturation-value 
of  it  given  in  Note  6,  P.  11.  Let  the  mixture  of  Pb(NO3)2  and 
NH4C1  stand  3  minutes ;  note  the  result,  and  explain  it.  Add 
to  the  mixture  2  cc.  more  of  the  3  n.  NH4C1  solution,  and  note 
and  explain  the  result. 

To  one  drop  of  the  test-solution  of  AgNOs  in  12  cc.  of  water 
in  a  test-tube  add  4  cc.  of  the  3  n.  NH4C1  solution.  Calculate 
the  normal  concentration  of  silver-ion  and  that  of  chloride-ion 
in  the  solution  at  the  moment  of  mixing  (before  the  precipitate 
has  separated),  assuming  that  the  salts  are  completely  ionized; 
and  find  the  corresponding  value  of  the  ion-concentration  prod- 
uct for  silver  chloride.  Calculate  also  the  saturation-value  of 
that  product  from  the  solubility  of  silver  chloride  given  in  the 
Table  of  Solubilities  in  the  Appendix;  and  determine  also  the 
limit  of  delicacy  of  this  test  for  silver  by  calculating  the  smallest 
number  of  milligrams  which,  if  present  in  the  solution,  could 
have  given  a  precipitate. 

Experiment  3.  —  Analysis  of  the  Silver-Group.  —  Study 
Table  III  (preceding  P.  n).  Mix  in  a  conical  flask  20  cc.  of 
the  test-solution  of  Pb(NO3)2  with  5  cc.  portions  of  the  test- 
solutions  of  AgNO3  and  Hg2(N03)2,  and  treat  the  mixture  by 
P.  11-13.  Read  the  Notes  on  P.  12-13. 


LABORATORY  EXPERIMENTS  5 

*  Treat  the  black  residue  left  by  NH4OH  by  P.  14.     Read  the 
Notes  on  P.  14. 

*Note.  —  Experiments  or  parts  of  experiments  preceded  by  an 
asterisk  may  be  omitted  in  brief  courses  on  the  subject  when  the  in- 
structor so  directs. 

Experiment  4.  —  Precipitation  by  Hydrogen  Sulfide.  —  To 
10  cc.  of  the  test-solution  of  Bi(NO3)3  add  5  cc.  of  HNOs,  4  cc. 
of  3  n.  NH4C1  solution,  and  80  cc.  of  water ;  and  treat  the  mix- 
ture as  described  in  the  second  and  third  sentences  of  P.  21. 
Read  Notes  3-5  of  P.  21. 

Note.  —  The  HNO»  and  NH4C1  are  added  and  the  mixture  is  diluted 
to  100  cc.,  so  as  to  have  the  conditions  the  same  as  those  prevailing  in 
actual  analyses. 

Experiment  5.  —  Effect  of  Acid  on  the  Precipitation  by  Hydrogen 
Sulfide.  —  Introduce  into  each  of  three  test-tubes  by  means  of  a 
dropper  3  drops  of  the  test-solution  of  Cd(NO3)2.  Add  to  the 
first  tube  i  cc.  of  HC1,  to  the  second  3  cc.  of  HC1,  and  to  the 
third  9  cc.  of  HC1.  Then  add  to  each  solution  enough  water  to 
make  the  volume  about  20  cc.,  and  pass  a  slow  current  of  H2S 
into  it  for  about  a  minute.  Repeat  the  last  test  (with  9  cc.  of 
HC1),  substituting  Cu(NO3)2  for  the  Cd(N03)2.  Calculate  the 
normal  concentration  of  the  HC1  in  each  tube,  and  record  and 
explain  the  results.  Study  Note  6,  P.  21. 

*  Experiment  6. — Precipitation  of  Arsenic  by  Hydrogen  Sul- 
fide. —  To  10  cc.  of  the  test-solution  of  HsAs04  add  5  cc.  of 
HN03,  4  cc.  of  3  n.  NH4C1  solution,  and  80  cc.  of  water.     Treat 
this  mixture  by  the  whole  of  P.  21,  omitting  the  final  filtration. 
Read  Notes  7  and  8,  P.  21. 

Experiment  7.  —  Effect  of  Oxidizing  Substances  on  Hydrogen 
Sulfide.  —  To  20  cc.  of  the  test-solution  of  Fe(N03)3  add  4  cc. 
of  3  n.  NH4C1  solution,  5  cc.  of  HN03,  and  70  cc.  of  water,  and 
pass  in  H2S  till  the  solution  is  saturated.  Repeat  this  experi- 
ment, substituting  20  cc.  of  the  test-solution  of  K2CrO4  (not  the 
K2Cr04  reagent)  for  that  of  the  Fe(N03)3.  Study  Notes  9  and 
10,  P.  21. 


6  LABORATORY  EXPERIMENTS 

Experiment  8.  —  Analysis  of  the  Copper-Group.  —  Study 
Table  V  (preceding  P.  31).  Mix  10  cc.  portions  of  the  test- 
solutions  of  Pb(N03)2,'Bi(NO3)3,  Cu(NO3)2,  and  Cd(NO3)2,  add 
5  cc.  of  HNO3,  4  cc.  of  3  n.  NH4C1  solution,  and  50  cc.  of  water, 
treat  the  mixture  by  the  first  paragraph  of  P.  21,  and  treat  the 
precipitate  so  obtained  by  P.  31-37.  Read  the  Notes  on  P.  31- 

37- 

Experiment  9.  —  Analysis  of  an  Unknown  Solution  for  the 
Copper-Group.  — «Ask  the  instructor  for  an  unknown  solution 
(Unknown  A)  containing  elements  of  the  copper-group,  and 
analyze  10  cc.  of  it  for  those  elements.  First  add  5  cc.  of  HN03 
and  4  cc.  of  3  n.  NBUCl  solution,  and  treat  the  mixture  by  the 
first  paragraph  of  P.  21.  Treat  the  precipitate  thus  obtained 
by  P.  31-37.  Estimate  the  quantities  present,  and  record  and 
report  the  results,  as  described  in  the  following  directions. 

Directions  for  Analyzing  Unknown  Solutions.  —  Estimate  the 
number  of  milligrams  of  any  constituent  present  from  the  size 
of  the  precipitate  obtained  in  the  Confirmatory  Test  or  in  the 
Procedure  preceding  it.  In  order  to  make  this  estimate  more 
accurate,  compare  it,  unless  the  precipitate  is  obviously  very 
large,  with  that  obtained  by  subjecting  a  known  quantity  of 
the  test-solution  directly  to  the  same  final  Procedure.  For  this 
purpose  use  of  the  test-solution  0.5  cc.  (measured  with  a  dropper) 
in  case  the  precipitate  is  small,  or  5  cc.  (measured  in  a  10  cc. 
graduate)  in  case  it  is  fairly  large,  or  both  volumes  in  separate 
tests  in  case  it  is  intermediate  in  size.  (Note  that  fifteen  medium- 
sized  drops  correspond  to  0.5  cc.,  and  that  0.5  cc.  of  the  test- 
solution  contains  5  mg.  of  the  constituent  to  be  tested  for.) 
Record  the  analyses  of  unknown  solutions  in  the  note-book  in 
three  columns  headed  Operations,  Observations,  Conclusions. 
Enter  the  operations  and  observations  in  the  same  brief  form 
employed  in  the  experiments  with  known  solutions.  In  the 
column  headed  Conclusions  insert  the  conclusions  that  may 
be  drawn  from  each  observation  as  to  the  presence  or  absence 
of  any  of  the  constituents  that  may  be  present  in  the  unknown 
solution;  and  give  the  estimate  made  of  the  number  of  milli- 


LABORATORY  EXPERIMENTS  ^ 

grams  present  in  10  cc.  of  the  solution.  The  chemical  equa- 
tions involved  need  not  be  written. 

After  the  record  has  been  written  up  completely  in  the  note- 
book, report  in  duplicate  the  results  of  the  analysis  to  the  in- 
structor in  the  form  shown  in  the  Note  below,  stating  not 
only  the  nature  of  the  constituents,  but  also  the  approximate 
quantities  of  them  found  present  in  the  10  cc.  of  solution. 
Quantities  less  than  5  mg.  may  be  reported  as  "  small  "  (s) ; 
those  from  5  to  50  mg.  as  "  medium  "  (m) ;  and  those  greater 
than  50  mg.  as  "  large  "  (/).  (It  is  to  be  noted,  since  one  gram 
of  a  non-metallic  solid  substance  is  ordinarily  taken  for  analysis, 
that  5  mg.  corresponds  to  the  presence  of  0.5%  and  50  mg.  to 
the  presence  of  5%  of  the  constituent  in  such  a  substance.) 
The  instructor  will  return  one  of  the  duplicate  reports  with  an 
entry  on  it  showing  the  quantities  of  the  various  constituents 
which  the  unknown  solution  actually  contained. 

The  correctness  of  the  results  obtained  in  the  analysis  of 
these  unknown  solutions  is  an  important  factor  in  determining 
the  grade  of  the  student.  As  the  unknowns  will  contain  as 
little  as  2  or  3  mg.  of  some  constituents,  satisfactory  results 
can  be  secured  only  by  careful  manipulation  and  intelligent 
following  of  directions. 

Note.  —  Cards  with  the  following  heading  are  conveniently  em- 
ployed for  the  reports  of  these  analyses  of  unknown  solutions,  and  of 
the  later  analyses  of  unknown  solid  substances ;  space  being  available 
below  the  heading  for  eight  or  ten  constituents  on  each  half  of  the 
card. 

NAME DATE 

UNKNOWN  No. . .  ...  GRADE  . . 


CONSTIT- 
UENT 

QUANT. 
FOUND 

QUANT.  1 
PRESENT 

CONSTIT- 
UENT 

QUANT. 
FOUND 

QUANT. 
PRESENT 

Experiment  10.  —  Behavior  of  Tin-Group  Elements  toward 
Hydrogen  Sulfide  and  Sodium  Sulfide.  —  Study  Table  IV  (pre- 
ceding P.  21).  To  5  cc.  of  water  in  each  of  four  test-tubes  add 


8  LABORATORY  EXPERIMENTS 

from  a  dropper  6  drops  of  the  test-solutions  of  HgCl2,  of  AsCla,  of 
SbCla,  and  of  H2SnCl6,  respectively.  Pass  H2S  into  each  tube  for 
half  a  minute.  Then  add  from  a  graduate  2  cc.  of  Na2S  reagent. 
Finally,  add  3  cc.  of  HC1  slowly  to  each  tube,  and  shake  the 
mixture.  Compare  these  precipitates  with  that  produced  by 
mixing  5  cc.  of  water,  2  cc.  of  Na2S  reagent,  and  3  cc.  of  HC1, 
and  shaking.  Read  Notes  2  and  3,  P.  22,  and  Notes  3-5,  P.  23. 

Note. — In  an  actual  analysis  the  analyst  decides,  whenever  possible, 
from  the  appearance  of  the  HC1  precipitate  whether  or  not  the  tin- 
group  is  present  in  significant  quantity.  Note  that  the  six  drops  of 
the  test-solutions  used  in  this  experiment  correspond  to  2  mg.  of  the 
constituent. 

Experiment  n.  —  Separation  of  the  Tin-Group  from  the  Copper- 
Group.  —  Refer  to  Table  IV.  To  a  mixture  of  5  cc.  portions  of 
the  test-solutions  of  Bi(N03)3,  HgCl2,  AsCla,  SbClg,  and  H2SnCl6 
add  5  cc.  of  HNO3,  4  cc.  of  3  n.  NH4C1  solution,  and  enough 
water  to  make  the  volume  ico  cc.  Treat  the  mixture  by  the 
first  paragraph  of  P.  21,  filter  with  the  aid  of  suction  (see  Note  2, 
P.  23),  and  treat  the  precipitate  by  P.  22,  using  10  cc.  of  Na2S 
reagent.  Reject  the  residue  of  Bi2S3,  and  treat  the  sulfide  solu- 
tion by  P.  23.  Treat  at  once  the  HC1  precipitate  obtained  in 
P.  23  as  described  in  Expt.  12.  Read  Notes  4-6,  P.  22,  and 
Notes  6-7,  P.  23. 

Experiment  12.  —  Analysis  of  the  Tin-Group.  — .Study  Table 
VI  (preceding  P.  41).  Treat  the  HC1  precipitate  of  the  tin- 
group  sulfides  obtained  in  Expt.  n  by  P.  41-47.  Read  the 
Notes  on  P.  41-47. 

Experiment  13.  —  Analysis  of  Unknown  Solutions  for  the 
Copper-  and  Tin-Groups.  —  Ask  for  an  unknown  solution  (Un- 
known B)  containing  elements  of  the  tin-group,  and  another 
unknown  solution  (Unknown  C)  containing  elements  of  the 
copper-group  and  tin-group,  and  analyze  10  cc.  of  each  of  them. 
First,  in  order  to  secure  the  proper  acid  concentration  for  the 
H2S  precipitation,  make  the  solution  exactly  neutral  by  adding 
to  it  NH4OH,  drop  by  drop,  till  it  no  longer  reddens  blue  litmus 
paper,  and  add  just  5  cc.  of  HNO3  and  enough  water  to  make 


LABORATORY  EXPERIMENTS  9 

the  volume  100  cc.  Then  treat  the  mixture  by  P.  21-23,  followed 
by  P.  41-47  in  the  case  of  Unknown  B,  and  by  P.  31-37  and 
P.  41-47  in  the  case  of  Unknown  C. 

Experiment  14.  —  Detection  of  Phosphate.  —  Pour  i  cc.  of 
the  test-solution  of  Ca3(P04)2  in  HN03  into  a  mixture  of  2  cc. 
of  HN03  and  2  cc.  of  (NH4)2MoO4  reagent,  and  heat  the  mixture 
to  60-70°.  Read  P.  50  and  the  Notes  on  it. 

Experiment  15.  —  Precipitation  of  the  Aluminum-  and  Iron- 
Groups  and  Solution  of  the  Group- Precipitate.  —  Treat  a  mixture 
of  10  cc.  portions  of  the  test-solutions  of  Co(NO3)2  and  of 
Fe(N03)3  by  P.  51  and  by  the  first  paragraph  of  P.  52,  omitting 
the  filtration  and  evaporation  at  the  end.  Refer  to  Table  VII 
(preceding  P.  51),  and  read  Note  i,  P.  51,  and  Notes  1-2,  P.  52. 

Experiment  16.  —  Behavior  of  Elements  of  the  Aluminum-  and 
Iron-Groups  toward  Ammonium  Hydroxide  and  Sulfide.  —  To  5  cc. 
portions  of  the  test-solutions  of  A1(NO3)3,  Cr(NO3)3,  Fe(N03)3> 
FeCl2,  Zn(NO3)2,  Mn(N03)2,  Ni(NO3)2,  and  Co(N03)2,  in 
separate  test-tubes  add  i  cc.  of  3  n.  NH4C1  solution  and  8-10 
drops  of  NH4OH,  and  note  the  result.  Then  add  2-3  cc.  more 
of  NH4OH.  Finally,  add  1-2  cc.  of  6  n.  (NH4)2S  solution  to 
each  tube.  Filter  out  the  NiS  precipitate,  and  boil  the  filtrate 
for  2  or  3  minutes.  Record  the  results  of  all  these  tests  in  a 
single  table,  so  as  to  show  what  effect  is  observed  and  what 
compound  is  formed  in  the  case  of  each  element  upon  the  addi- 
tion of  each  reagent.  Study  the  results,  refer  to  Table  VII, 
and  read  Notes  2-5  and  8-10,  P.  51. 

Experiment  17.  —  Behavior  of  Elements  of  the  Aluminum-  and 
Iron-Groups  toward  Sodium  Hydroxide  and  Peroxide.  —  To 
separate  5  cc.  portions  of  the  test-solutions  named  in  Expt.  16 
add  8-10  drops  of  6  n.  NaOH  solution,  and  note  the  result. 
Then  add  2-3  cc.  more,  and  again  note  the  result.  Finally, 
to  each  of  the  mixtures  add  gradually  from  a  dry  graduate 
(without  using  paper)  0.2-0.3  cc.  of  Na202  powder,  and  heat 
it  to  boiling.  Record  all  the  results  in  a  single  table  as  in  Expt. 
1 6.  Study  the  results,  refer  to  Table  VII,  and  read  Notes  3-7, 
P.  52. 


io  LABORATORY  EXPERIMENTS 

Experiment  18.  —  Analysis  of  the  Aluminum-Group.  —  Study 
Table  VIII  (preceding  P.  53).  Treat  a  mixture  of  io  cc.  portions 
of  the  test-solutions  of  A1(NO3)3,  Zn(NO3)2,  and  Cr(NO3)3  by  the 
second  paragraph  of  P.  52  and  by  P.  53-57.  Read  the  Notes 
on  P.  53-57. 

Experiment  19.  —  Analysis  of  the  Iron-Group  :  Separation  of 
Manganese  and  Iron.  —  Study  Table  IX  (preceding  P.  61), 
considering  only  the  case  where  "  phosphate  is  absent."  Treat 
a  mixture  of  io  cc.  portions  of  the  test-solutions  of  Mn(N03)2, 
Fe(NO3)3,  Co(NO3)2,  and  Ni(N03)2  and  of  a  2  cc.  portion  of 
that  of  Zn(N03)2  by  the  second  paragraph  of  P.  52 ;  and  treat 
the  precipitate  thereby  obtained  by  P.  61,  62,  and  63.  Treat 
the  nitrate  containing  the  zinc,  cobalt,  and  nickel  as  described 
in  Expt.  20.  Read  the  Notes  on  P.  61,  62,  and  63. 

*  Experiment  20.  —  Analysis  of  the  Iron-Group  :   Separation 
of  Zinc,  Cobalt,  and  Nickel.  —  Study  Table  X  (preceding  P.  66). 
Treat  the  nitrate  obtained  in  Expt.  19  by  P.  65-68.     Read  the 
Notes  on  P.  65-68. 

Note.  —  In  brief  courses  this  experiment  may  be  omitted.  And 
in  the  unknowns  subsequently  given  for  analysis  nickel  (but  not  cobalt) 
may  be  included,  it  being  detected  by  passing  H2S  into  the  filtrate  from 
the  NH4OH  precipitate  (P.  63)  as  described  in  the  first  sentence  of 
P.  65 ;  and  zinc  need  be  tested  for  only  in  the  analysis  of  the  aluminum- 
group. 

Experiment  21.  —  Analysis  of  an  Unknown  Solution  for  Ele- 
ments of  the  Aluminum-  and  Iron-Groups.  —  Ask  the  instructor 
for  an  unknown  solution  for  this  purpose  (Unknown  D),  and 
treat  io  cc.  of  it  by  P.  51-57,  61-63,  and  65-68,  first  diluting  it 
with  90  cc.  of  water. 

*  Experiment  22.  —  Precipitation  of  Alkaline-Earth  Elements 
by   Ammonium    Hydroxide   in   the   Presence   of  Phosphate.  — 
Heat  0.2  cc.  of  Cas^O^a  powder  with  io  cc.  of  water;    then 
add  5  cc.  of  HNO3,  and  boil  the  mixture  for  one  minute.    To 
the  solution  add  NH4OH  till  the  mixture,  after  shaking,  smells 
of  it ;  filter,  and  add  i  cc.  of  (NH4)2CO3  reagent  to  the  filtrate. 
Repeat  the  experiment,  using  0.2  cc.  of  CaC03  powder  in  place 


LABORATORY  EXPERIMENTS  II 

of  the  Ca3(PO4)2.  Explain  in  the  note-book  why  the  calcium 
is  precipitated  by  NH4OH  in  one  case  and  not  in  the  other. 
Read  Notes  6-7,  P.  51,  and  Note  8,  P.  52. 

*  Experiment  23. — Modification  of  the  Analysis  of  the  Iron- 
Group  in  the  Presence  of  Phosphate  for  the  Purpose  of  Detecting 
Alkaline-Earth  Elements.  —  Mix  together  10  cc.  portions  of  the 
test-solutions  of   Fe(NO3)3   and   of   Co(NO3)2,  and  of  that  of 
Ca3(PO4)2  in  HN03.     Treat  this  solution  by  the  last  paragraph 
of  P.  64  and  by  P.  65.     To  the  nitrate  obtained  in  P.  65  add 
2-3  cc.  of  (NH4)2CO3  reagent.     Read  the  Notes  on  P.  64. 

*  Experiment    24.  —  Analysis   of  an  Unknown  Solution  for 
Elements  of  the  Aluminum-  and  Iron-Groups  in  the  Presence  of 
Phosphate.  —  Ask  the  instructor  for  an  unknown  solution  for 
this  purpose  (Unknown  E),  and  treat  10  cc.  of  it  by  P.  51-57 
and  6 1-68. 

Experiment  25.  —  Precipitation  of  the  Alkaline- Earth  Group. 
-  To  2  cc.  of  the  test-solution  of  Mg(NO3)2  add  8  cc.  of  water 
and  2  cc.  of  (NH4)2CO3  reagent,  and  shake  the  mixture  for 
about  a  minute;  then  add  5  cc.  of  (NH4)2CO3  reagent  and 
5  cc.  of  95%  ethyl  alcohol,  and  shake  for  a  minute  more.  Re- 
peat the  experiment,  using  the  test-solution  of  Ca(NO3)2  in 
place  of  that  of  Mg(N03)2,  and  filtering  out  any  precipitate 
before  adding  the  second  portion  of  (NH4)2C03  reagent.  Read 
P.  71  and  the  Notes  on  it. 

Experiment  26.  —  Analysis  of  the  Alkaline- Earth  Group.  — 
Study  Table  XI  (preceding  P.  71).  Mix  in  a  100  cc.  flask  3  cc. 
portions  of  the  test-solutions  of  BaCl2,  Sr(NO3)2,  Ca(NO3)2, 
and  Mg(N03)2 ;  and  treat  the  solution  by  the  second  para- 
graph of  P.  71,  followed  by  P.  72-79.  Read  the  Notes  on  P. 
72-79. 

Experiment  27.  —  Analysis  of  the  Alkali-Group  by  the  Shorter 
Less  Exact  Method.  —  Read  the  General  Discussion  and  study 
Table  XII  (preceding  P.  81).  Mix  10  cc.  of  the  test-solution  of 
KN03  and  10  cc.  of  that  of  NaNO3  with  4  cc.  of  3  n.  NH4C1 
solution  and  5  cc.  of  (NH4)2CO3  reagent,  and  treat  the  mixture 
by  P.  81-83.  Read  the  Notes  on  P.  81-83. 


12  LABORATORY  EXPERIMENTS 

*  Experiment  28.  —  Analysis  of  the  Alkali-Group  by  the  Exact 
Method.  —  Study  Table  XIII  (preceding  P.  85).     Prepare  the 
same  mixture  as  in  Expt.  27,  add  to  it  i  cc.  of  the  test-solution  of 
Na2SO4,  and  treat  it  by  P.  85-89.     Read  the  Notes  on  P.  85-89. 

Experiment  29. —  Analysis  of  an  Unknown  Solution  for 
Elements  of  the  Alkaline- Earth  and  Alkali-Groups.  —  Ask  the 
instructor  for  an  unknown  solution  for  this  purpose  (Unknown 
F),  and  analyze  10  cc.  of  it  by  P.  71-79  and  P.  81-83, *or  by 
P.  71-79  and  P.  85-89. 

*  Experiment  30.  —  Detection  of  Ammonium.  —  Treat  0.2  g. 
of  NH4C1  by  P.  91.     Read  the  Notes  on  P.  91. 

*  Experiment  31.  —  Determination  of  the  State  of  Oxidation  of 
Certain  Elements  Existing  in  Two  Such  States.  — Read  the  General 
Discussion  and  study  Table  XIV   (preceding  P.   91).     Treat 
0.2  g.  of  finely  powdered  Fe304  (ferro-ferric  oxide)  as  described 
in  the  first  two  paragraphs  of  P.  92.    Read  the  Notes  on  P.  92. 

Experiment  32.  —  Analysis  of  Unknown  Solutions  for  All  the 
Basic  Constituents.  —  Ask  the  instructor  for  two  unknown  solu- 
tions for  this  purpose  (Unknowns  G  and  H),  and  analyze  10  cc. 
of  each  of  them  by  P.  11-92.  Before  precipitating  with  H2S, 
exactly  neutralize  the  solution  with  NH^OH  and  add  5  cc.  of 
HC1. 


LABORATORY  EXPERIMENTS  13 

DETECTION  OF  ACIDIC   CONSTITUENTS  IN  NON-IGNEOUS   PRODUCTS 

Experiment  33.  —  Preparation  of  a  Solution  for  Detecting  the 
Acidic  Constituents.  —  Read  the  General  Discussion  of  the 
Detection  of  Acidic  Constituents  (preceding  P.  100).  Treat 
i  g.  of  the  solid  test-mixture  consisting  of  30%  of  BiOCl,  30% 
of  Fe2(SO4)3,  30%  of  NaNO3,  and  10%  of  Na2SO3  by  the  first 
paragraph  of  P.  101,  but  using  only  10  cc.  of  the  Na2C03  solution 
and  diluting  the  filtrate  to  only  12  cc.  Reserve  the  filtrate  for 
use  in  Expts.  34,  36,  and  37.  Read  the  Notes  on  P.  101. 

Experiment  34.  —  Detection  of  the  Chloride-Group.  —  Study 
the  first  column  of  Table  XV  (preceding  P.  102).  Treat  a 
portion  of  the  Na2C03  solution  prepared  in  Expt.  33  by  P.  102. 
Read  Note  i,  P.  102. 

Experiment  35.  —  Behavior  of  Acidic  Constituents  toward 
Silver  Nitrate.  —  To  separate  2  cc.  portions  of  the  test-solutions 
of  Na2S,  NaCN,  KI,  KBr,  NaCl,  KSOsT,  NaNO2,  Na2HPO4, 
and  K2Cr04,  add  a  few  drops  of  AgN03  solution,  and  then  i  cc. 
of  HN03.  Read  Notes  2-4,  P.  102. 

Experiment  36.  —  Detection  of  the  Sulfate-Group.  —  Study  the 
second  column  of  Table  XV.  Treat  a  portion  of  the  Na2COs 
solution  prepared  in  Expt.  33  by  P.  103.  Read  the  Notes  on 
P.  103. 

Experiment  37.  —  Detection  of  Oxidizing  and  Reducing  Con- 
stituents. —  Study  the  last  two  columns  of  Table  XV.  Treat 
separate  portions  of  the  Na2C03  solution  prepared  in  Expt.  33 
by  P.  104  and  105.  Read  the  Notes  on  these  Procedures. 

Experiment  38.  —  Identification  of  Constituents  by  the  Group- 
Reagents.  —  Ask  the  instructor  for  two  unknown  solutions 
(Unknowns  I  and  J),  each  of  which  will  contain  only  one  acidic 
constituent.  Add  to  5  cc.  of  each  of  these  solutions  5  cc.  of  3  n. 
Na2CO3  solution,  and  treat  portions  of  the  mixture  by  P.  102- 
105.  On  the  basis  of  the  results  of  these  tests,  taking  into 
account  the  colors  of  the  precipitates,  report  what  possibilities 
exist  as  to  the  nature  of  the  single  constituent  present  in  each 
solution. 


14  LABORATORY  EXPERIMENTS 

Experiment  39.  —  Analysis  of  the  Chloride-Group.  —  Study 
Table  XVI  (preceding  P.  106).  Mix  2  cc.  portions  of  the  test- 
solutions  of  Na2S,  NaCN,  K4Fe(CN)6,  KI,  KBr,  NaCl,  KSCN, 
and  KC1O3.  Add  to  the  mixture  5  cc.  of  3  n.  Na^COs  solution, 
and  treat  it  by  P.  106,  107,  and  the  first  paragraph  of  P.  108 ; 
reserving  the  Ni(NOs)2  and  AgNO3  precipitates  for  use  in  Expts. 
40  and  41.  Read  the  Notes  on  P.  106  and  107,  and  Notes  1-2, 
P.  108. 

*  Experiment   40.  —  Detection  of   the    Different   Cyanides.  — 
Study  Table  XVII  (preceding  P.   109).     Treat  the  Ni(N03)2 
precipitate  obtained  in  Expt.  39  by  the  first  three  paragraphs  of 
P.  109.     Read  the  Notes  on  P.  109. 

Experiment  41.  —  Detection  of  Thiocyanate  and  the  Different 
Halides.  —  Study  Table  XVIII  (preceding  P.  no).  Treat  the 
AgNO3  precipitate  obtained  in  Expt.  39  by  P.  no.  Read  the 
Notes  on  P.  no. 

*  Experiment  42.  —  Detection  of  Hypochlorite  and  of  Chlorate. 
—  Treat  0.5  g.  of  bleaching  powder  by  the  second  paragraph  of 
P.  108.     Read  Notes  3  and  4,  P.  108. 

Experiment  43.  —  Analysis  of  an  Unknown  Solution  for  Con- 
stituents of  the  Chloride-Group.  —  Ask  the  instructor  for  an  un- 
known solution  (Unknown  K)  for  this  purpose.  To  5  cc.  of  it 
add  i  cc.  of  3  n.  Na2CO3  solution.  Treat  the  mixture  by  P.  106, 
107,  and  the  first  paragraph  of  P.  108.  Treat  the  AgNOs 
precipitate  by  P.  no.  *  Treat  the  Ni(N03)2  precipitate  by 
P.  109. 

Experiment  44.  —  Analysis  of  the  Sulfate-Group.  —  Study 
Table  XIX  (preceding  P.  in).  Mix  2  cc.  portions  of  the  test- 
solutions  of  Na2S,  Na2S04,  Na2SO3,  and  NaF.  Add  to  the 
mixture  5  cc.  of  3  n.  Na2CO3  solution,  and  treat  it  by  P.  111-112. 
Read  the  Notes  on  P.  111-112. 

Experiment  45.  —  Detection  of  Nitrate  or  Nitrite.  —  Study  the 
first  two  columns  of  Table  XX  (preceding  P.  113).  Add  i  cc. 
of  the  test-solution  of  NaN02  to  2  cc.  of  3  n.  Na2CO3  solu- 
tion, and  treat  the  mixture  by  P.  113.  Read  the  Notes  on 
P.  113- 


LABORATORY  EXPERIMENTS  15 

Experiment  46.  —  Detection  of  Nitrite.  —  Add  3  drops  of  the 
test-solution  of  NaN02  to  i  cc.  of  3  n.  Na2CO3  solution,  and  treat 
the  mixture  by  P.  114.  Read  the  Notes  on  P.  114. 

Experiment  47.  —  Detection  of  Borate.  —  Refer  to  the  third 
column  of  Table  XX.  Evaporate  to  dryness  in  small  casseroles 
a  5-drop  portion  and  a  2-cc.  portion  of  the  test-solution  of  NaBO2 ; 
and  to  each  of  the  residues  add  3  cc.  of  3  n.  Na2CO3  solution. 
Treat  these  mixtures,  and  also  3  cc.  of  3  n.  Na2CO3  solution  to 
which  no  NaB02  is  added,  by  P.  115.  Read  the  Notes  on  P.  115. 

*  Experiment  48.  —  Detection  of  Ar senate  and  Ar senile. — 
Study  the  last  column  of  Table  XX.  Treat  by  P.  116  a  mixture 
of  2  cc.  of  the  test-solution  of  H3As04,  2  cc.  of  the  test-solution 
of  NaAs02,  and  4  cc.  of  3  n.  Na2CO3  solution.  Read  the  Notes 
on  P.  116. 

Experiment  49.  —  Analysis  of  an  Unknown  Solution  for  the 
Acidic  Constituents  Tested  for  in  Sodium  Carbonate  Solution.  — 
Ask  the  instructor  for  an  unknown  solution  (Unknown  L)  for 
this  purpose.  To  10  cc.  of  it  add  20  cc.  of  3  n.  Na2C03  solution, 
and  treat  portions  of  the  mixture  by  P.  102-116. 


1 6  LABORATORY  EXPERIMENTS 

DETECTION   OF    ACIDIC  CONSTITUENTS   IN    NATURAL    SUBSTANCES 
AND   IGNEOUS   PRODUCTS 

Experiment  50.  —  Detection  of  Sulfate,  Carbonate,  Sulfide,  and 
Cyanide.  —  Study  Table  XXI  (preceding  P.  121).  Treat  0.5  g. 
of  the  test-mixture  consisting  of  60%  of  gypsum  (CaS04  •  2  H2O), 
20%  of  marble  (CaC03),  10%  of  pyrite  (FeS2),  and  10%  of 
KCN,  as  directed  in  the  first,  second,  and  fourth  paragraphs 
of  P.  121.  Read  the  Notes  on  P.  117  and  121. 

Experiment  51.  —  Detection  of  Chloride,  Fluoride,  and  Borate. 
—  Study  Table  XXII  (preceding  P.  122).  Treat  i  g.  of  the 
test-mixture  consisting  of  5%  of  salt  (NaCl),  5%  of  fluorite 
(CaF2),  5%  of  borax  (Na2B4O7),  and  85%  of  fine  sand  by  P.  122, 
omitting  the  confirmatory  test  for  fluoride  and  the  comparison 
of  the  borate  color  with  standards.  Read  the  Notes  on  P.  122. 


LABORATORY  EXPERIMENTS  17 

PREPARATION   OF    THE  SOLUTION    FOR    THE    DETECTION  OF    BASIC 

CONSTITUENTS    AND    THE    COMPLETE    ANALYSIS    OF 

UNKNOWN   SUBSTANCES 

Experiment  52.  —  Indications  of  Certain  Constituents  A  forded 
by  the  Closed-Tube  Test.  —  Treat  separately  by  the  first  para- 
graph of  P.  i  samples  of  the  following  substances :  NaC2H302, 
Cu(N03)2  •  3H2O,  NI^Cl,  and  FeS2.  Read  the  Notes  on  P.  i, 
and  write  the  equations  expressing  the  reactions  involved  (except 
in  the  case  of  the  NaC2H3O2). 

General  Directions  for  Complete  Analyses.  —  Record  the 
results  in^the  note-book  and  report  them  in  the  way  explained 
in  the  Directions  for  Analyzing  Unknown  Solutions  (preceding 
Expt.  10).  In  the  case  of  solid  substances,  not  only  report  the 
constituents  found  present  and  the  quantities  of  them  estimated 
to  be  contained  in  one  gram  of  the  substance,  but  state  also  the 
compound  or  compounds  of  which  the  substance  seems  to  be 
mainly  composed.  Submit  the  reports  to  the  instructor  in 
duplicate  on  cards  like  those  described  in  the  Directions  for 
Analyzing  Unknown  Solutions. 

The  quantity  of  a  solid  substance  taken  for  the  analysis  should 
be  weighed  (within  o.i  g.)  on  a  rough  balance,  not  guessed  at 
nor  estimated  by  volume. 

In  analyses  where  a  number  of  different  precipitates  and 
filtrates  are  successively  obtained,  any  of  these  that  are  set 
aside,  even  temporarily,  should  be  distinctly  labeled,  in  order 
to  avoid  mistakes.  A  convenient  method  of  doing  this  is  to 
mark  on  the  label  simply  the  Procedure  by  which  the  precipitate 
or  nitrate  is  next  to  be  treated ;  thus  the  H2S  precipitate  would 
be  marked  P.  22,  and  the  filtrate  from  it  P.  51.  The  final  tests 
for  any  element  may  be  marked  Test  for  Pb,  Test  for  Al,  etc. 

Experiment  53.  —  Analysis  of  Non-Metallic  Substances  Dis- 
solved by  Water  or  Dilute  Acid.  —  Ask  the  instructor  for  two 
such  non-igneous  substances  (Unknowns  i  and  2),  and  treat 
samples  of  each  of  them  as  directed  in  P.  i .  Read  the  Notes  on 
P.  2  ;  and  study  the  General  Statement  relating  to  Solubilities  in 
the  Appendix. 


1 8  LABORATORY  EXPERIMENTS 

Experiment  54.  —  Analysis  of  Non-Metallic  Substances  Dis- 
solved only  by  Concentrated  Acids.  —  Study  the  upper  part  of 
Table  I  (preceding  P.  2).  Ask  the  instructor  for  two  non- 
igneous  products  and  two  natural  substances  or  igneous  products 
dissolved  only  by  concentrated  acids  (Unknowns  3,  4,  5,  and  6), 
and  treat  samples  of  each  of  them  as  directed  in  P.  i.  If  there 
is  any  residue  undissolved  at  the  end  of  P.  3,  disregard  it  in  these 
practice  analyses.  Read  the  Notes  on  P.  3. 

Experiment  55.  —  Analysis  of  Alloys.  —  Ask  the  instructor 
for  two  alloys  (Unknowns  7  and  8),  and  treat  a  sample  of  each 
as  directed  in  P.  4,  followed  by  P.  21-78.  Read  the  Notes  on 

P.  4- 

*  Experiment  56.  —  Analysis  of  Natural  Substances  or  Igneous 
Products    Not  Completely  Dissolved  by   Treatment  with    Nitric 
and   Hydrochloric  Acids.  —  Study  the  lower  part  of  Table  I 
(preceding  P.  2).     Ask  the  instructor  for  two  such  substances 
(Unknowns  9  and  10).     Treat  i  g.  of  each  of  them  by  P.  2,  3,  5, 
and  6,  followed  by  P.  11-89.     Treat  fresh  samples  of  each  of 
the  substances  by  P.  121  and  122.     Read  the  Notes  on  P.  5  and  6. 

Ask  the  instructor  for  another  such  substance  (Unknown  n). 
Treat  i  g.  of  it  by  P.  2  and  3.  Treat  the  solution  obtained  in 
P.  3  by  P.  21-89,  and  the  residue  undissolved  in  P.  3  as  directed 
in  P.  7.  Treat  fresh  samples  of  the  substance  by  P.  121  and 
122.  Read  the  Notes  on  P.  7  and  123. 

*  Experiment  57.  —  Analysis  of  Non-Igneous  Products  Con- 
taining Organic  Matter.  —  Ask  the  instructor  for  such  a  substance 
(Unknown  12),  and  treat  a  sample  of  it  by  P.  i,  another  sample 
as  directed  in  P.  8,  and  a  third  sample  as  directed  in  P.   100. 
Read  the  Notes  on  P.  8. 

*  Experiment  58.  —  Analysis  of  Solutions.  —  Ask  the  instructor 
for  an  aqueous  solution  which  is  a  trade  preparation  (Unknown 
13),  and  treat  it  as  described  in  P.  9.     Read  the  Notes  on  P.  9. 


QUESTIONS   ON  THE  EXPERIMENTS 

DETECTION    OF   THE    BASIC    CONSTITUENTS 

Experiment  i. — Separation  of  the  Basic  Constituents  into  Groups. — 
i.  In  precipitating  the  silver-group  in  an  actual  analysis  could  the  NH4Cl 
be  replaced  by  NaCl?  byHCl?  Why  or  why  not  ? 

2.  If  the  NH4C1  were  not  added,  what  would  happen  to  the  silver  in  the 
subsequent  parts  of  the  experiment? 

3.  Of  the  five  basic  constituents  present  in  the  mixture  why  is  silver  the 
only  one  that  is  precipitated  by  NH4C1? 

4.  If  enough  H2S  were  not  used  to  precipitate  all  the  copper,  how  would 
it  behave  on  the  subsequent  addition  of  NH4OH  and  (NH4)2S? 

5.  What  is  the  first  reaction  that  takes  place  when  NH4OH  is  added  to 
the  filtrate  from  the  H2S  precipitate? 

6.  What  would  happen  to  the  (NH4)2S  if  it  were  added  directly  to  the 
filtrate  from  the  H2S  precipitate,  without  first  adding  NH4OH? 

7.  What  happens  to  the  (NH4)2S  when  the  filtrate  from  the  (NH4)2S 
precipitate  is  evaporated  ? 

8.  If  all  the  basic  constituents  had  been  present  hi  the  original  mixture 
used  for  this  experiment,  what   ones   would  have  been  precipitated  by 
(a)  NH4C1,  (ft)  H2S,  (c~)  NH4OH  and  (NH4)2S,  (d)  (NH4)2CO3?     (e)  What 
ones   would   have   been   left   with   the   potassium   in    the   filtrate  from 
the  (NH4)2COS  precipitate? 

Experiment  2.  —  Precipitation  of  the  Silver-Group .  Principles  Relating 
to  Equivalents,  Concentration,  and  Solubility- Effect.  —  i.  What  would  be 
meant  by  the  statement  that  a  certain  quantity  of  lead  nitrate  is  equivalent 
to  a  certain  other  quantity  of  ammonium  chloride? 

2.  In  making  up  one  liter  of  3  n.  NH4C1,  how  many  grams  of  the  salt 
should  be  weighed  out,  and  how  much  water  should  be  added  to  it?     (For 
the  atomic-weight  values  needed  in  answering  this  and  other  questions  see 
the  table  in  the  Appendix.) 

3.  In  making  up  a  liter  of  6  n.  H2S04,  how  many  cubic  centimeters  of 
95%  sulfuric  acid  (s.  g.,  1.84)  should  be  used,  and  how  much  water  should 
be  added  to  it? 

4.  Approximately  how  many  cubic  centimeters  of  3  n.  NH4C1  solution 
would  be  required  to  precipitate  500  mg.  of  silver?     (Calculate  first  the 
number  of  equivalents  corresponding  to  500  mg.  Ag.)     (Since  i  g.  of  the 
unknown  substance  is  ordinarily  taken  for  the  analysis  for  basic  constituents, 
500  mg.  is  as  large  a  quantity  of  any  element  as  is  likely  to  be  present.) 

19 


20  QUESTIONS  ON  THE  EXPERIMENTS 

5.  In  general,  how  many  cubic  centimeters  of  a  i  n.  reagent  must  be 
added  to  react  with  500  mg.  of  an  element  which  has  an  equivalent  weight 
of  100?  of  50?  of  20?    (This  principle  should  be  frequently  applied  through- 
out the  System  of  Analysis ;  with  its  aid  the  number  of  cubic  centimeters' of 
any  reagent  theoretically  required  to  precipitate  the  maximum  amount  of 
an  element  likely  to  be  present  can  quickly  be  estimated.) 

6.  Estimate  the  volume  of  3  n.  Na2CO3  solution  required  to  precipitate 
500  mg.  of  calcium.    Estimate  the  volume  of  6  n.  NH4OH  required  to  pre- 
cipitate 500  mg.  of  iron  when  present  in  the  form  of  Fe(N03)3- 

7.  Explain  by  the  solubility-product  principle  why  PbQ2  is  less  soluble 
in  a  solution  containing  NH4C1  than  in  pure  water.     (This  question  may 
be  answered  by  shortening  the  complete  explanation  given  in  Note  6,  P.  n, 
as  follows:  "  In  any  dilute  sol'n  satur.  with  PbCl2,  (Pb++)X(Cl-)2=satur. 
value.     NH4C1  added  to  such  a  sol'n  causes,  owing  to  its  ionization  into 
NH4+and  C1-,  an  increase  in  (C1-),  and  therefore  raises  (Pb++)  X  (Cl-)2 
above  the  saturation  value,  so  that  PbCl2  ppts."   (All  other  questions  as  to 
the  effect  of  one  substance  on  the  solubility  of  another  substance  should  be 
answered  in  a  similar  way.    Always  consider  the  effect  which  the  added  sub- 
stance may  have  on  the  concentration  of  each  of  the  ions  of  the  salt  with  which 
the  solution  is  saturated,  and  state  the  reason  for  any  such  effect.) 

8.  Calculate  by  the  solubility-product  principle,  from  the  fact  that  the 
solubility  of  PbCl2  in  water  at  20°  is  0.070  normal,  what  its  solubility  would 
be  in  a  solution  0.40  normal  in  chloride-ion  (which  is  approximately  the 
chloride-ion  concentration  in  the  0.63  normal  NH4C1  solution).    Assume  the 
PbCl2  to  be  completely  ionized. 

9.  From  the  result  found  in  Question  8  calculate  how  many  milligrams 
of  lead  would  have  to  be  present  in  15  cc.  of  water  at  20°,  hi  order  that 
precipitation  of  PbCl2  may  result  on  adding  to  it  4  cc.  of  3  n.  NH4C1  solution. 
(Owing  to  the  tendency  to  form  supersaturated  solutions,  a  much  larger 
quantity  of  lead  may  actually  be  present  before  precipitation  occurs.) 

10.  Compute  the  number  of  milligrams  of   mercurous  mercury  that 
must  be  present  in  15  cc.  of  water  in  order  that  it  could  precipitate  on  the 
addition  of  4  cc.  of  3  n.  NH4C1  solution,  taking  into  account  the  facts  that 
Hg2Cl2  has  a  solubility  of  0.000002  normal  and  that  its  molecule  dissociates 
into  one  Hg2++  ion  and  two  Cl~  ions. 

Experiment  3.  —  Analysis  of  the  Silver-Group  —  i.  Why  is  a  consider- 
able excess  of  NH4C1  added  in  precipitating  the  silver-group?  (The  term 
excess  signifies  the  quantity  added  beyond  the  equivalent  quantity  theoret- 
ically required  to  produce  the  reaction  in  question.) 

2.  Explain  why,  in  apparent  contravention  of  the  solubility-product 
principle,  a  large  excess  of  NH4C1  would  increase  the  solubility  of  the  silver- 
group  chlorides.  (This  difference  in  the  solubility-effect  of  a  slight  excess 


QUESTIONS  ON  THE  EXPERIMENTS  21 

and  of  a  large  excess  of  a  reagent  is  a  phenomenon  frequently  met  with  in 
analytical  chemistry,  and  it  commonly  arises  from  the  same  kind  of  influence 
as  is  here  involved.) 

3.  What  might  happen  if  the  solution  of  the  nitrates  to  which  the  NH4C1 
solution  is  added  had  a  much  larger  volume  than  1 5  cc.  ? 

4.  Why  is  the  precipitate  of  the  silver-group  chlorides  washed  with 
dilute  HC1  rather  than  with  water?     Could  it  be  washed  with  dilute  NH4C1 
solution  equally  well? 

5.  The  solubility  of  AgCl  at  100°  is  0.022  g.  per  liter.    How  many  milli- 
grams of  silver  might  be  lost  if  100  cc.  of  boiling  water  were  used  for  extract- 
ing the  lead  from  the  chloride  precipitate? 

6.  With  what  other  reagents  besides  K2Cr04  might  the  hot-water  ex- 
tract be  tested  for  lead?     (See  the  Table  of  Solubilities  in  the  Appendix.) 
What  advantage  does  K2CrO4  have  over  each  of  these  other  reagents  with 
respect  to  the  delicacy  or  to  the  characteristicness  of  the  test  ? 

7.  Explain  by  the  solubility-product  principle  why  the  formation  of  the 
complex  salt  Ag(NH3)2+Cl~  causes  AgCl  to  be  much  more  soluble  in  NH4OH 
solution  than  in  water.     (Answer  in  accordance  with  the  Note  on  Question  7, 
Expt.  2.) 

8.  Formulate  the  mass-action  expression  for  the  equilibrium  between 
the  complex  cation  Ag(NH3)2+  and  its  constituents.     Show  by  reference  to 
this  expression  and  the  solubility-product  principle  why  the  addition  of 
HNOs  causes  AgCl  to  be  precipitated  out  of  its  solution  in  NH4OH. 

Experiments  4  and  5.  —  Precipitation  by  Hydrogen  Sulfide.  —  i.  In  the 
precipitation  of  bismuth  caused  by  diluting  the  solution  with  water,  what 
ion-concentration  product  comes  into  consideration?  What  must  be  true 
of  its  value  in  order  that  bismuth  may  be  precipitated?  Explain  why 
decreasing  the  HN03  concentration  decreases  the  quantity  of  bismuth 
that  remains  in  solution. 

2.  In  precipitating  with  H2S  what  is  the  reason  for  adding  5  cc.  of  HN03 
and  diluting  the  solution  to  loocc.  ?    Why  not  use  less  acid  and  thus  avoid 
all  risk  of  failing  to  precipitate  the  elements  of  the  copper-  and  tin-groups  ? 

3.  In  passing  H2S  into  a  Cu(N03)2  solution,  at  what  stage  in  the  process 
does  the  solution  after  shaking  begin  to  smell  of  the  gas? 

4.  Write  the  chemical  equation  expressing  the  precipitation  of  copper- 
ion  by  dissolved  H2S.    By  formulating  the  corresponding  mass-action 
expression,  show  how  the  concentration  of  copper-ion  remaining  unpre- 
cipitated  is  related  to  the  H2S  concentration  and  the  H+  ion  concentration. 
(Note  that  the  same  would  be  true  of  any  bivalent  metal  ion.) 

5.  What  principle  determines  how  the  concentration  of  H2S,  that  is,  the 
quantity  of  it  dissolved  by  a  unit-volume  of  water,  varies  with  its  partial 
pressure?    What  would  its  partial  pressure  be  in  a  mixture  made  by  mixing 
i  volume  of  H2S  with  4  volumes  of  air  at  a  pressure  of  i  atmosphere? 


22  QUESTIONS  ON  THE  EXPERIMENTS 

6.  Why  would  a  larger  quantity  of  an  element  have  to  be  present  in 
order  to  give  a  precipitate  if  the  solution  were  treated  with  H2S  in  an  open 
beaker,  instead  of  in  the  closed  flask? 

7.  Give  two  reasons  why  a  larger  quantity  of  an  element  would  have  to 
be  present  to  give  a  precipitate  if  the  solution  were  saturated  with  H2S 
at  80°,  instead  of  at  20°. 

8.  The  solubility  (in  equivalents  per  liter)  of  freshly  precipitated  ZnS 
in  water  is  about  1000  times  as  great  as  that  of  CdS.     Calculate  by  the  prin- 
ciples discussed  in  Note  6,  P.  21,  the  ratio  of  the  hydrogen-ion  concentrations 
at  which  the  precipitation  of  cadmium  and  zinc  will  barely  take  place  when 
the  concentration  of  each  of  them  has  any  definite  value  (for  example, 
o.oooi  equivalents  per  liter). 

9.  The  pressure-volume  relations  of  perfect  gases  are  expressed  by  the 
equation  pv/T=&2  N,  when  the  pressure  p  is  in  atmospheres,  the  volume  » 
in  cubic  centimeters,  and  the  temperature  T  in  centigrade-degrees  on  the 
absolute  scale,  and  when  the  quantity  of  the  gas  is  N  gram-molecular- 
weights.     Calculate  the  number  of  cubic  centimeters  of  H2S  at  25°  required 
to  precipitate  500  mg.  of  copper. 

Experiment  6.  —  Precipitation  of  Arsenic  by  Hydrogen  Sulfide.  —  i.  By 
what  reaction  is  the  HNOs  destroyed  when  the  arsenic  solution  to  which 
HC1  has  been  added  is  evaporated  to  dryness?  Could  HC1  be  destroyed 
in  the  same  way  by  evaporating  a  solution  of  chloride  with  HNOs  ? 

2.  If  the  HNOs  were  not  so  destroyed,  what  would  happen  when  the 
H2S  is  passed  into  the  hot,  strongly  acid  solution  ? 

3.  What  difference  in  ionization  relations  accounts  for  the  facts  that, 
unlike  the  other  elements,  arsenic  in  the  form  of  HsAsO*  is  only  very  slowly 
precipitated  from  a  cold,  weakly  acid  solution,  and  that  its  precipitation  is 
greatly  promoted  by  increasing  the  HC1  concentration? 

4.  Write  the  series  of  reactions  that  occur  when  H2S  is  passed  into  a 
dilute  HC1  solution  of  H3AsO4. 

Experiment  7.  —  Effect  of  Oxidizing  Substances  on  Hydrogen  Sulfide.  — 
i.  What  substances  besides  ferric  salts  might  be  present  which  would 
liberate  sulfur  from  H2S  ? 

2.  Write  the  equation  expressing  the  reaction  between  each  of  these 
substances  and  H2S,  balancing  the  equations  by  the  method  described  hi 
Note  10,  P.  21. 

3.  Write  by  the  same  method  the  equations  expressing  the  oxidation  of 
H2S,  in  one  case  to  sulfur  and  in  another  to  H2SO4,  by  hot,  fairly  concentrated 
HNO3,  assuming  that  the  HN03  is  reduced  to  NO. 

Experiment  8.  —  Analysis  of  the  Copper-Group.  —  i.  Make  a  table 
showing  briefly  in  the  first  column  the  chemical  operations  involved  in 
analyzing  a  solution  for  lead  and  bismuth  (by  P.  21  and  P.  31-35),  and  show- 


QUESTIONS  ON  THE  EXPERIMENTS        23 

ing  in  a  second  and  in  a  third  column  the  behavior  of  these  two  elements  in 
each  operation.  "  Behavior  "  in  this  and  later  questions  means  both  the 
effect  observed  and  the  chemical  compound  produced.  Thus,  the  first 
two  operations  and  the  results  of  them  hi  the  case  of  lead  should  be  entered 
as  follows : 

Operation  Behavior  of  Lead 

Saturate  with  H2S.  Black  ppt.  of  PbS. 

Boil  with  3  n.  HNOi.  Ppt.  diss.,  forming  colorless 

sol'n  of  Pb++(NCV)2. 

2.  Make  a  similar  table  showing  the  operations  involved  in  analyzing  a 
solution  for  copper  and  cadmium  (by  P.  21,  31,  32,  34,  36,  and  37),  and  show- 
ing the  behavior  of  these  elements. 

3.  Explain  by  the  solubility-product  principle  the  fact  that  CuS,  which 
is  only  slightly  soluble  in  hot  dilute  HC1,  dissolves  readily  in  hot  dilute 
HNOa  of  the  same  concentration. 

4.  Write  the  equation  expressing  the  dissolving  of  CuS  in  3  n.  HNOs. 

5.  Why  may  a  black  residue  be  left  undissolved  by  HNO8? 

6.  Why  does  the  evaporation  with  H2SO4  convert  the  salts  present  into 
sulfates?     Could  sulfates  be  converted  into  nitrates  by  evaporating  with  a 
large  excess  of  HNOi? 

7.  Explain  with  reference  to  the  solubility-product  principle  why  PbSO4 
is  much  more  soluble  in  dilute  HNOj  than  in  water.     (H2SO4  in  dilute  solu- 
tion is  dissociated  almost  completely  into  H+  and  HSO4~ ;  but  the  latter  ion 
is  only  to  a  moderate  extent  dissociated  into  H+  and  SO4=.) 

8.  What  effect,  as  compared  with  that  of  HNO3,  would  HC1  have  on 
the  solubility  of  PbSO4?    What  effect  would  KNO3  have?     Give  reasons. 
(K2S04  in  dilute  solution,  like  other  unibivalent  salts,  but  unlike  H2SO4,  is 
almost  completely  dissociated  into  the  simple  ions,  K+  and  SO4~,  with 
formation  of  only  a  small  proportion  of  the  intermediate  ion,  KSO4~.) 

9.  Explain  by  the  solubility-product  principle  why  the  fact  that  PbAc2 
is  a  slightly  ionized  substance  should  cause  PbSO4  to  dissolve  much  more 
readily  in  NH4Ac  solution  than  in  water. 

10.  Would  one  expect  PbCrO4  also  to  be  more  soluble  in  NH^c  solution 
than  in  water?    Why  or  why  not?  If  so,  why  does  PbCr04  precipitate  from 
the  same  NH^c  solution  that  dissolves  PbSO4? 

11.  Explain  with  the  aid  of  the  mass-action  expressions  involved  why 
Cu(OH)2,  a  substance  very  slightly  soluble  in  water,  is  not  precipitated  by 
the  NH4OH.     Show  that  the  presence  of  the  (NH4)2SO4  in  the  solution 
must  diminish  the  tendency  of  it  to  precipitate. 

12.  If  the  lead  were  not  removed  by  the  addition  of  H2SO4,  would  it  be 
precipitated  as  Pb(OH)2  on  the  addition  of  an  excess  of  NH4OH?    What 


24  QUESTIONS  ON  THE  EXPERIMENTS 

knowledge  in  regard  to  lead  compounds  would  enable  one  to  predict  whether 
or  not  this  precipitation  would  take  place? 

13.  Write  the  equations  expressing  the  formation  of  Na2SnO2  from 
SnCl2  and  NaOH ;    also  that  expressing  the  spontaneous  decomposition  of 
Na2SnO2  into  Sn  and  Na2SnO3 ;  also  that  expressing  its  action  on  Bi03H3. 

14.  Lead  hydroxide,  like  Sn(OH)2,  is  an  amphoteric  substance.    What 
is  meant  by  this  statement?    What  experiments  might  be  made  to  deter- 
mine whether  it  is  true? 

15.  The  presence  of  bismuth  in  the  NH4OH  precipitate  can  be  confirmed 
by  dissolving  and  reprecipitating  as  BiOCl.     Describe  a  procedure  by 
which   this  confirmatory  test  could  be  made  so  as  to  be  delicate,  taking 
into  account  the  fact  that  BiCOl,  though  very  slightly  soluble  in  water, 
increases  rapidly  in  solubility  as  the  concentration  of  acid  in  the  solution 
increases. 

16.  If  (NH4)2S  be  added  to  the  NH4OH  solution,  both  CuS  and  CdS 
are  precipitated.     What  does  this  show  as  to  the  degree  of  dissociation  of 
the  complex  ammonia  ions  of  these  elements  into  the  simple  ions? 

17.  If  K4Fe(CN)6  be  added  to  the  NH4OH  solution  (without  neutralizing 
it  with  HAc) ,  no  precipitate  results  unless  a  fairly  large  quantity  of  copper 
is  present.    Explain  this  fact. 

18.  With  the  aid  of  the  Table  of  Specific  Reduction-Potentials  in  the 
Appendix  show  by  computation  of  the  actual  reduction-potentials  whether 
copper  would  be  precipitated  till  its  concentration  became  as  small  as 
o.oooi  formal  by  metallic  Pb  (as  it  is  by  metallic  Fe),  assuming  its  ions 
attained  in  the  process  a  concentration  of  o.i  formal. 

19.  Show  by  similar  computations  whether  lead  and  bismuth  (if  not 
previously  removed  by  the  H2S04  and  NH4OH)  would,  like  copper,  be  almost 
quantitatively  precipitated  by  metallic  Fe. 

20.  CdS,  though  very  slightly  soluble  in  water,  is  much  more  soluble 
in  it  than  is  CuS  (as  illustrated  by  Expt.  5).     Outline  a  series  of  experi- 
ments that  might  be  made  to  determine  whether  on  this  fact  could  be  based 
a  procedure  by  which  i  mg.  of  cadmium  could  be  detected  in  the  presence 
of  500  mg.  of  copper. 

Experiments  10  and  n.  —  Separation  of  the  Tin-  and  Copper-Groups.  — 
i.  Write  chemical  equations  expressing  the  two  stages  of  the  hydrolysis 
of  Na2S.  Explain  by  the  ionic  theory  and  the  mass-action  law  why  this 
hydrolysis  takes  place,  taking  account  of  the  fact  that  water  is  ionized  to 
a  slight  extent  into  H+  and  OH~. 

2.  A  solution  made  by  dissolving  crystals  of  Na2S  •  9  H2O  in  water  has 
a  strong  alkaline  reaction  to  litmus,  a  pronounced  slippery  feel,  and  scarcely 
any  odor.  What  conclusions  may  be  drawn  from  these  facts  as  to  the 
degree  to  which  each  stage  of  the  hydrolysis  has  taken  place? 


QUESTIONS  ON  THE  EXPERIMENTS  25 

3.  Explain  by  the  mass-action  law  how  the  presence  of  NaOH  in  the 
Na2S  reagent  decreases  the  hydrolysis  of  the  salt. 

4.  Write  equations  expressing  the  action  of  HC1  on  the  Na2S  and  on 
the  Na2S2  present  in  the  reagent. 

5.  What  is  the  main  purpose  of  having  a  certain  proportion  of  Na2S2 
in  the  reagent  ? 

6.  Write  equations  showing  the  behavior  of  SnCl2  and  of  H2SnCl6  when 
treated  in  succession  with  H2S  by  P.  21,  with  Na2S  reagent  by  P.  22,  and 
with  HC1  by  P.  23. 

7.  Explain  by  the  mass-action  law  why  the  addition  of  HC1  to  a  solu- 
tion of  Na2SnSs  causes  the  precipitation  of  SnS2. 

8.  In  what  respects  is  the  separation  of  the  elements  of  the  copper  group 
from  those  of  the  tin-group  by  the  Na2S  reagent  imperfect  ? 

9.  If  (NH4)2S  is  used  for  the  separation,  mercury  remains  as  HgS  almost 
completely  with  the  elements  of  the  copper-group,  instead  of  passing  into 
the  sulfide  solution.     Suggest  an  explanation  of  this  striking  difference  in 
behavior,  taking  into  account  that  the  only  differences  in  the  two  solutions 
are  those  arising  from  the  fact  that  NH4OH  is  a  slightly  ionized  base  and 
NaOH  a  largely  ionized  one. 

Experiment  12.  —  Analysis  of  the  Tin-Group.  —  i.  Describe  the  dif- 
ferences in  the  solubilities  of  the  sulfides  of  mercury,  arsenic,  antimony,  and 
tin  on  which  the  separation  of  these  elements  from  one  another  is  based. 

2.  In  treating  the  sulfides  with  12  n.  HC1  why  does  much  more  As2S8 
dissolve  if  the  solution  be  allowed  to  boil?    Why  does  the  solution  boil 
at  so  low  a  temperature  as  50-60°  ? 

3.  Write  by  the  method  described  in  Note  10,  P.  21,  the  equations  ex- 
pressing the  action  of  HC1  on  KClOg  by  which  C12  is  produced  and  that  by 
which  C102  is  produced. 

4.  Suggest  a  reason  why  in  the  confirmatory  test  for  mercury  the  presence 
of  HC1  tends  to  prevent  the  immediate  reduction  of  Hg2Cl2  to  Hg. 

5.  Explain  why  the  C12  set  free  by  the  addition  of  the  KC103  causes  the 
HgS  to  dissolve  even  in  the  dilute  HC1. 

-  6.  What  is  the  expression  for  the  solubility-product  in  the  case  of 
MgNH4AsO«?  Why  does  it  dissolve  readily  in  HC1?  (See  the  Table  of 
lonization-Values  in  the  Appendix.) 

7.  Why  does  the  hydrolysis  of  this  salt  increase  its  solubility?    Why  is 
that  hydrolysis  decreased  by  an  excess  of  NH4OH  ?    How  is  the  hydrolysis 
affected  by  the  presence  of  NH4C1?    Would  NH4C1  affect  the  solubility  in 
any  other  way? 

8.  What  is  a  saturated  solution?  a  supersaturated  one?    By  what  treat- 
ments can  a  precipitate  be  made  to  separate  from  a  supersaturated  solution  ? 

g.   With  the  aid  of  the  Table  of  Specific  Reduction-Potentials  in  the 


26  QUESTIONS  ON  THE  EXPERIMENTS 

Appendix  state  what  metals  other  than  Sn  would  precipitate  Sb  from  a 
solution  i  n.  in  HC1.  What  is  the  objection  to  using  a  more  strongly  re- 
ducing metal  in  place  of  tin? 

10.  State  what  might  be  expected  to  happen  on  placing  metallic  Pb  in  a 
solution  o.i  n.  in  SnCl2  and  i  n.  in  HC1. 

11.  Show  what  metals  (if  any)  other  than  Sb  could  be  used  for  reducing 
tin  from  the  stannic  to  the  stannous  state  in  chloride  solution,  without  pre- 
cipitating nearly  all  of  the  tin  as  metal. 

12.  What  is  the  significance  of  the  fact  that  the  specific  reduction- 
potential  of  tin  has  a  positive  value,  and  that  of  antimony  a  negative  one? 
How  might  this  fact  be  made  the  basis  of  a  method  of  separating  the  two 
metals  if  they  had  been  precipitated  together? 

13.  Write  chemical  equations  expressing  the  formation  of  NaOBr  from 
Br2  and  NaOH,  its  spontaneous  decomposition  into  NaBr  and  NaBrOj, 
and  the  action  of  it  on  metallic  As. 

14.  How  does  the  confirmatory  test  for  tin  with  HgClj  differ  in  type 
from  the  usual  method  of  detecting  an  element? 

15.  Explain  with  reference  to  the  reduction-potentials  involved  why  in 
i  n.  HC1  solution  a  small  proportion  of  SnCl2  reduces  HgCl2  to  Hg2Cl2, 
and  an  excess  of  SnCl2  reduces  it  to  Hg. 

1 6.  Predict  from  the  reduction-potentials  involved  whether  the  presence 
of  iron  in  the  antimony  used  as  reagent,  and  hence  of  FeCl2  in  the  solution, 
would  cause  reduction  of  the  HgCl2  to  Hg2Cl2. 

17.  State  how  any  antimony  that  remained  as  Sb2Ss  in  the  residue  un- 
dissolved  by  the  treatment  with  12  n.  HC1  in  P.  41  would  behave  in  the 
subsequent  procedures  (P.  42  and  44).     (In  answering  questions  of  this 
type  any  needed  information  not  otherwise  available  can  usually  be  secured 
by  simple  test-tube  experiments.) 

18.  State  how  any  mercury  and  any  arsenic  that  went  into  solution  in 
the  12  n.  HC1  in  P.  41  would  behave  in  the  subsequent  procedures  (P.  45-47). 

19.  State  how  any  antimony  that  failed  to  be  precipitated  as  Sb2S3  in 
P.  45  would  behave  in  P.  47. 

Experiment  14.  —  Detection  of  Phosphate.  —  i.  Show  by  the  mass- 
action  law  why  a  large  concentration  of  hydrogen-ion  promotes  the  formation 
of  the  complex  phosphomolybdate  anion,  noting  that  MoO3  is  the  anhydride 
of  H2Mo04  and  that  the  concentration  of  the  latter  must  determine  that  of 
the  former  when  equilibrium  is  reached. 

2.  What  might  be  expected  to  be  the  effect  of  NI^OH  on  the  yellow 
precipitate  of  ammonium  phosphomolybdate  ? 

3.  Give  a  plausible  reason  why  heating  promotes  the  precipitation  of 
ammonium  phosphomolybdate,  even  though  it  is  probably  more  soluble 
in  hot  solutions. 


QUESTIONS  ON  THE  EXPERIMENTS  27 

Experiment  15.  —  Precipitation  of  the  Aluminium-  and  Iron-Groups.  — 
i.  In  an  actual  analysis  how  many  cubic  centimeters  of  NH4OH  would  be 
required  to  neutralize  the  5  cc.  of  HN03  that  are  added  before  precipitating 
withH,S? 

2.  How  much  more  NH4OH  would  be  needed  to  neutralize  the  solution 
if  500  mg.  Cu  had  been  present  in  the  form  of  Cu(NO3)2  in  the  solution  pre- 
cipitated by  H2S?     (In  all  such  calculations  of  the  volume  of  the  reagent 
needed,  first  reduce  the  weight  of  the  constituent  from  grams  to  equivalents.) 

3.  How  does  testing  the  vapors  above  the  solution  with  PbAc2  paper 
show  that  an  excess  of  (NH4)2S,  a  non- volatile  salt,  has  been  added? 

4.  If  in  an  actual  analysis  the  mixture  containing  NH4OH  and  (NH4)2S 
were  allowed  to  absorb  C02  from  the  air  before  filtering,  what  difference 
would  it  make? 

5.  Why  is  the  (NH4)2S  precipitate  treated  first  with  cold  HC1?    Why  is 
HNO3  subsequently  added? 

Experiments  16  and  17.  —  Behavior  toward  Ammonium  and  Sodium 
Hydroxides.  —  i.  Which  elements  are  soluble:  (a)  in  excess  of  NH4OH  (in 
the  presence  of  NH4Cl),but  not  in  excess  of  NaOH ;  (6)  in  excess  of  NaOH, 
but  not  of  NH4OH  (in  presence  of  NH4C1) ;  (c)  in  excess  both  of  NH4OH  and 
of  NaOH ;  (d)  neither  in  excess  of  NaOH  nor  of  NH4OH  (in  presence  of 
NH4C1)  ? 

2.  What  are  the  explanations  of  the  four  typical  cases  (a),  (&),  (c),  (d), 
referred  to  in  the  preceding  question  ? 

3.  Could  the  hydroxide  of  an  element  which  does  not  form  a  complex 
ammonia  cation  be  soluble  in  NH4OH  and  not  in  NaOH  ?     Could  a  hydroxide 
be  readily  soluble  in  NaOH  and  yet  no  more  soluble  in  NH4OH  than  in 
water  ? 

4.  Show  by  formulating  and  combining  the  mass-action  equation  for 
the  solubility-product  of  A1(OH)3  dissociating   as   a  base  into  Al+++  and 
OH~  ions  and  the  mass-action  equation  for  the  formation  of  A102~  out  of 
Al+++  and  OH~  ions  that  the  quantity  of  aluminum  dissolved  (as  A102~) 
in  the  presence  of  a  base  is  proportional  to  the  OH~  concentration  in  the 
solution. 

5.  Name  all  the  elements  that  form  ammonia  complexes  in  all  the  groups 
thus  far  considered.    What  can  be  said  as  to  the  position  of  these  elements 
in  the  periodic  system  ?     (Refer  to  a  text-book  of  Inorganic  Chemistry.) 

6.  If  in  an  actual  analysis  no  precipitate  is  obtained  on  the  addition  of 
NH4OH,  what  conclusion  may  be  drawn? 

7.  Which  of  the  hydroxides  precipitated  by  NH4OH  undergo  no  change 
on  addition  of  (NH4)2S? 

8.  What  must  be  the  explanation  of  the  facts  that  these  trivalent  hy- 
droxides are  not  converted  into  sulfides  and  that  the  bivalent  elements  of 
these  groups  are  precipitated  as  sulfides  but  not  as  hydroxides? 


28  QUESTIONS  ON   THE  EXPERIMENTS 

9.  Which  of  the  hydroxides  precipitated  by  NaOH  undergo  change  on 
the  addition  of  Na2O2,  and  into  what  compound  is  each  of  these  hydroxides 
converted? 

10.  What  substances  are  produced  by  the  action  of  Na/^  on  water? 
Experiment  18.  —  Analysis  of  the  Aluminum-Group.  —  i.   In  separating 

the  aluminum  from  the  chromium  and  zinc  with  NH4OH  in  P.  53,  what 
would  be  the  harm  of  adding  too  small  an  excess?  What  of  adding  too  large 
an  excess? 

2.  H2Si03  (in  hydrated  form)  dissolves  somewhat  in  solutions  of  dilute 
acids  and  readily  in  those  of  largely  ionized  bases,  but  is  precipitated  from 
solutions  with  small  concentrations  of  H+  or  OH~  ions  (such  as  prevails  in 
a  solution  of  NH4OH  and  ammonium  salt).     If  H2SiO3  were  present  in  the 
solution,  state  how  it  would  behave  hi  P.  51,  52,  53,  and  54. 

3.  What  is  meant  by  adsorption?    How  is  it  illustrated  hi  the  con- 
firmatory test  for  aluminum? 

4.  What  is  meant  by  a  basic  salt?    Why  is  the  precipitate  produced 
by  NaaCOs  with  zinc  salts  called  basic  zinc  carbonate?    How  is  BiOCl 
related  to  a  basic  bismuth  chloride? 

5.  In  dissolving  the  sulfides  in  HC1  and  HNO3  in  P.  52  a  little  H2S04  is 
usually  formed.    Would  this  be  expected  to  have  any  effect  on  the  test  for 
chromate  with  PbAc2  in  P.  57  ? 

6.  Make  a  table  (like  that  described  in  Question  i  on  Expt.  8)  showing 
the  operations  involved  and  the  behavior  of  the  chromium  and  zinc  La 
analyzing  a  dilute  HNO3  solution  of  ZnCrO4,  beginning  with  the  H2S  pre- 
cipitation (P.  21)  and  continuing  through  the  analysis  of  the  aluminum- 
group  (P.  51-57).     At  the  foot  of  the  table  write  all  the  chemical  equations 
involved. 

Experiments  19  and  20.  —  Analysis  of  the  Iron-Group.  —  i.  What 
are  the  oxides  of  manganese  corresponding  to  its  three  stages  of  oxidation 
occurring  in  P.  61  and  62?  What  is  the  valence  of  manganese  in  each  of 
these  oxides?  How  do  they  differ  with  respect  to  the  formation  of  salts 
with  acids  and  with  bases? 

2.  Make  a  table  (like  that  described  in  Question  i  on  Expt.  8)  showing 
the  operations  involved  and  the  behavior  cf  manganese  in  analyzing  a  dilute 
HN03  solution  of  CaMnO4,  beginning  with  the  H2S  precipitation  and  con- 
tinuing through  the  final  test  for  manganese  (thus  involving  P.  21,  51,  52, 
61,  and  62).     Write  also  all  the  chemical  equations  involved. 

3.  Why  is  a  large  excess  of  NH4OH  added  hi  precipitating  the  iron  in 
P.  63? 

4.  Why  is  it  necessary  to  test  for  zinc  hi  the  analysis  of  the  iron-group? 

5.  Why  may  zinc  be  precipitated  by  NaOH  and  Na202  in  the  first  treat- 
ment (hi  P.  52),  and  yet  not  be  precipitated  by  them  in  the  second  treatment 
(in  P.  66)? 


QUESTIONS  ON  THE  EXPERIMENTS  29 

6.  When  the  original  Na2O2  precipitate  is  so  small  that  zinc  need  not  be 
tested  for  in  the  iron-group,  how  may  P.  66  be  simplified  ? 

7.  What  happens  to  Zn(NO3)2  and  to  Co(NO8)2  when  they  are  ignited 
separately,  and  when  an  intimate  mixture  of  them  is  ignited  as  in  the  con- 
firmatory test  for  zinc  ? 

8.  Write  the  chemical  equations  expressing  the  steps  by  which  CoClj 
may  be  considered  to  be  converted  into  KsCo(N02)j. 

9.  It  is  to  be  noted  that  cobalt  is  oxidized  to  the  cobaltic  state,  even 
by  fairly  strong  oxidizing  agents  like  Na2O2  or  HNO2,  only  when  there  is 
produced  a  very  slightly  soluble  cobaltic  compound  (such  as  Co(OH)s  in 
the  presence  of  NaOH),  or  a  complex  salt  (like  the  cobaltinitrite).    How 
must  these  conditions  affect  the  concentration  of  the  simple  Co+++  ion  in 
the  solution  ?    And  how  must  this  affect  the  reduction-potential  of  Co++ 
to  Co+++  and  the  tendency  of  cobalt  to  change  from  the  cobaltous  and  to 
cobaltic  state? 

10.  In  what  two  ways  may  the  fact  that  nickel  is  not  oxidized  under 
the  same  conditions  be  accounted  for? 

11.  Name  all  the  elements  thus  far  considered  which  in  any  state  of 
oxidation  form  colored  compounds  in  solution.    What  can  be  said  as  to  the 
position  of  these  elements  in    he  periodic  system? 

12.  State  how  each  of  the  following  elements  would  behave  if  it  were 
retained  in  the  precipitate  produced  by  H2S  in  P.  21  and  that  precipitate 
were  subjected  to  the  Procedures  for  the  analysis  of  the  copper-  and  tin- 
groups  :    (a)  iron  ;    (b}  aluminum  ;    (c)  zinc. 

13.  State  how  each  of  the  following  elements  would  behave  if  it  remained 
in  the  filtrate  from  the  H2S  precipitate  and  were  subjected  to  the  Pro- 
cedures for  the  precipitation  and  analysis  of  the  aluminum  and  iron  groups : 
(a)  copper ;  (b)  lead ;  (c]  arsenic.     (Cu(OK)2  is  not  soluble  in  an  excess  of 
NaOH ;   Pb(OH)2  is  soluble  in  excess  of  NaOH,  but  is  converted  into  in- 
soluble Pb02  by  Na202 ;  lead  is  not  precipitated  as  Pb02  by  HC103  in  HNO, 
solution.) 

Experiments  22  and  23.  —  Analysis  of  the  Aluminum-  and,  Iron-Groups 
in  the  Presence  of  Phosphate.  —  i.  What  bearing  would  the  fact  that  the 
original  substance  dissolved  in  water  with  a  neutral  or  alkaline  reaction 
to  litmus  have  on  the  possibility  of  alkaline-earth  elements  being  precipi- 
tated by  NH4OH  when  phosphate  is  present? 

2.  What  difference  would  it  make  in  this  conclusion  if  the  original  sub- 
stance dissolved  in  water  with  an  acid  reaction?     Give  an  example  of  a 
solid  substance  or  mixture  containing  alkaline-earth  phosphate  which  would 
so  dissolve. 

3.  If  in  P.  50  phosphate  has  been  found  present  and  in  P.  51  NH4OH 
has  given  a  precipitate,  explain  whether  it  would  be  necessary  to  test  for 
alkaline-earth  elements  in  the  filtrate  from  the  (NH4)2S  precipitate. 


30  QUESTIONS  ON  THE  EXPERIMENTS 

4.  If  in  P.  50  phosphate  has  been  found  present  and  in  P.  51  NH4OH 
produced  no  precipitate  but  (NH4)«S  did  so,  explain  whether  it  would  be 
necessary  to  provide  for  detecting  alkaline-earth  elements  in  the  analysis 
of  the  iron-group.    Name  a  substance  or  mixture  which  would  conform  to 
these  conditions. 

5.  In  the  test  for  iron  with  K4Fe(CN)6  why  must  HNO3  and  C12  first  be 
removed  by  evaporation  ? 

6.  By  formulating  the  mass-action  expression  for  the  hydrolysis  of 
FeAc3  show  how  the  ratio  of  the  concentration  of  Fe(OH)3  to  that  of  Fe+++ 
is  related  to  the  concentrations  of  HAc  and  Ac~  in  the  solution ;  and  state 
what  this  shows  as  to  the  best  conditions  for  securing  complete  precipitation 
of  the  iron.     State  what  limitation  makes  somewhat  difficult  the  realiza- 
tion of  these  conditions. 

7.  What  must  be  the  explanation  of  the  fact  that  the  phosphate  combines 
with  the  ferric  iron  rather  than  with  one  of  the  bivalent  elements  when 
both  are  present? 

8.  Describe  as  in  Question  i,  Expt.  8,  the  behavior  which  a  solution  of 
Ca3(P04)2  in  HN03  would  show  when  submitted  to  the  operations  involved 
in  the  precipitation  and  separation. of  the  aluminum  and  iron  groups  (P.  51- 
57,  61-68). 

Experiment  25.  —  Precipitation  of  the  Alkaline- Earth  Group.  —  i.  What 
does  this  experiment  show  as  to  the  precipitation  by  (NH4)2C03:  (a)  of 
magnesium,  and  (6)  of  the  other  alkaline-earth  elements  (which  all  behave 
like  calcium)  ? 

2.  Why  would  a  reagent  consisting  of  NH4HCO3  not  be  suitable  for  the 
separation?    Why  is  it  advantageous  to  have  more  NH3  present  than 
corresponds  to  the  neutral  salt  (NH4)2C03  ? 

3.  Why,  in  order  to  secure  complete  precipitation  of  the  magnesium,  is 
it  necessary  to  shake  the  mixture  and  let  it  stand  for  a  considerable  time? 
Does  this  change  the  solubility  of  the  precipitate  ? 

4.  To  what  is  the  action  of  the  alcohol  in  diminishing  the  solubility 
primarily  due  —  a  change  in  ionization  or  a  change  in  the  medium  as  a 
solvent  ? 

5.  In  some  schemes  of  analysis  (NH4)2CO3  is  used  under  other  con- 
ditions for  separating  barium,  strontium,  and  calcium  from  magnesium. 
What  experiments  would  one  make  in  developing  such  a  procedure  and 
testing  its  effectiveness? 

Experiment  26.  —  Analysis  of  the  Alkaline- Earth  Group.  —  i.  In  order 
to  make  a  separation  of  i  mg.  of  barium  from  500  mg.  of  strontium,  what 
must  be  the  concentration  of  CrO4~,  stated  with  reference  to  the  saturation- 
values  of  the  ion-concentration  products  of  BaCrO4  and  SrCrO4? 

2.   What  would  have  to  be  true  of  the  ratio  of  these  two  saturation- 


QUESTIONS  ON  THE  EXPERIMENTS  31 

values  in  older  that  this  separation  maty  be  possible?  What  n  the  actual 
ratio  of  these  two  values?  (See  the  Table  of  Sombffities  in  the  Appendix.) 

J.    Writ*  t ho  ma«*-anHnn  **pr»€«inn«  tnr  flw  ^yiJBhriMMi  «rf  tW  tiMrtJMi 

by  which  chromate-ion  is  converted  into  hydrodhromate-ion,  and  of  fiat 
by  which  hydrochromate-ion  is  converted  into  oichroaiaie-WB,  Show  by 
these  expressions  what  determines  the  relative  proportions  of  the  first  two 
of  these  ions  in  any  solution,  and  what  determines  the  concemrtxalion  of 

':..-.-."---..:.  .-.  =.-;/  -.  ;:;;- 

tions  in  order  to  secure  the  right  concentration  of  CrO«~  in  the  a 

5.  Why  is  the  second  KiCrO*  precipitate  obtained 

test  for  barium  more  conclusive  evidence  of  its  presence  than  the  first 
KjCrO*  precipitate? 

6.  On  addition  of  XH/)H  in  P.  74  what  rl*-™^!  change  Games  the 
change  in  color  from  orange  to  yellow?    Why  from  a  mass-action  stand- 
point does  the  addition  of  NHdOH  cause  this  change  to  take  place?    Why 

7.  FTplain  with  reference  to  the  saturation-values  of  the  ioA-conceB- 
tration  products  why  the  carbonate-oxalate  mixture  used  in  P.  75  cuuwtte 
SrCrO*  into  SrCOj  (rather  than  into  SiC-O^ .  and  CaCrOb  into  C&C&t 
(rather  than  into  CaCOj). 

S.  \VTiy  does  the  fact  that  HgC,0,  is  a  sfightry  ionixed  salt  cane  the 
::.-..'  :  :  __•.-  _:_,~_  :;  iiniziri  ".i.t  it_;ii".  .:  :ir  ;z.^..i:t  :tti:  :;: 
calcimn? 

9.  Why  does  CaCA  react  with  dflole  H^SO^  but  not  with  HAc? 

10.  How  does  the  confirmatory  test  for  nlrmm  distinguish  it  from 
barium  and  strontium,  which  form  much  less  «"h«mr  suhates?    How  does 


::      1  .  .'.  .  -  :--~  -.-.-.•--.  :-t  :  :T  •:.;  .-.--.-;  :  :y  -~  '•    ::bt:  mr^i: 
of  a  compound  dosehr  analogous  to  MgXHsPOt? 

12.  \\~hy  is  the  production  of  a  uminilatc  with  XasHPQ*  in  the  con- 
firmatory test  for  magnesium  (in  P.  79)  more  comcterwe  evidence  of  its 
presence  than  the  production  of  the  first  XasHPO*  |»imii|Miiate  (m  P.  ~S|? 

13.  How  would  each  of  the  alkaline-earth  Hrmrnrs  behave  in  the  sdb- 
5-e-:  •_:--.   ~-    .....-:?   ::  -.':.-   gr:_;   ::  :ii  ele-ti:  -trt  :..:    :.-:.::t.;.    ::;- 
:;:::.^:  i  :r.  :he  :::;•.:  ;'_L.:S: 

Experiments  27  and  2S.  —  Afdrsis  oj  tike  AIAoH-Gnrnp.  —  i.  H  the 
i~r-  .-     -     r       -   .--  -::   :•:  —  r".e:ely 


2.  Why  is  the  separation  of  potasExum  and  sodium  by  the  HQQi  i 
satisfactory  when  these  Hrmrnts  are  present  as  rMnrirtrs  or  aitxates,  bat 
not  when  they  are  present  as  suHates?    Why  b  it  saHMactnry  when  they 

ire  7  :;-«<-: 


32  QUESTIONS  ON  THE  EXPERIMENTS 

3.  What  is  implied  by  the  statement  that  Na3Co(N02)6  is  a  complex 
salt  in  solution? 

4.  How  might  a  solution  of  Na3Co(N02)6  be  prepared,  judging  from  the 
experience  previously  obtained  in  the  test  for  cobalt? 

5.  What  are  the  formulas  of  antimonic  acid,  pyroantimonic  acid,  and 
metantimonic  acid?    How  are  they  related  to  each  other?    What  other 
element  forms  a  similar  series  of  acids  ? 

Experiment  31.  —  Determination  of  the  State  of  Oxidation  of  Certain 
Elements  forming  Basic  Constituents.  —  i.  Make  a  table  showing  all  of 
these  elements  which  exist  hi  two  or  more  states  of  oxidation,  the  valence  of 
these  elements  in  each  of  these  states,  and  the  corresponding  ions  in  the 
form  of  which  the  elements  mainly  exist  in  solution. 

2.  Show  by  the  method  described  in  the  second  paragraph  of  Note  10, 
P.  21,  that  the  valences  of  the  elements  in  the  various  anions  included  in 
the  table  are  those  attributed  to  the  elements.     Show  whether  or  not 
chromium  is  in  the  same  state  of  oxidation  in  CrO4=  and  Cr207~. 

3.  State  what  indications,  if  any,  as  to  the  state  of  oxidation  of  each  of 
the  elements  existing  in  two  or  more  states  may  be  obtained  during  the 
course  of  the  analysis  for  basic  constituents. 


QUESTIONS  ON  THE  EXPERIMENTS  33 

DETECTION    OF   THE   ACIDIC   CONSTITUENTS 

Experiment  33.  —  Preparation  of  the  Sodium  Carbonate  Solution. — 
i.  State  what  action  boiling  Na2C03  solution  has  on  each  of  the  following 
substances :  PbCl2,  Cu(As02)2)  A1(OH)3,  NI^Cl,  CaCrO4,  and  CuS.  Write 
the  chemical  equations  involved. 

2.  Name  the  important  kinds  of  substances  that  are  not  much  acted 
upon  by  boiling  Na2CO3  solution. 

3.  With  the  aid  of  the  explanation  and  data  given  in  Note  7,  P.  101, 
calculate  how  many  milligrams  of  BaSO4  would  actually  be  converted  into 
BaCO3  at  20°  by  25  cc.  of  3  n.  Na2C03  solution  (assuming  that  the  Na2CO3 
and  Na2S04  in  the  solution  have  the  same  percentage  ionization).     Calcu- 
late how  many  milligrams  would  be  converted  if  only  the  equivalent  quantity 
of  Na2CO3  were  required. 

4.  With  aid  of  the  above  explanation  and  the  Table  of  Solubilities  in 
the  Appendix,  calculate  the  equilibrium  conditions  that  would  result  on 
treating  PbSO4  and  PbCr04  with  Na2C03  solution  at  20°.     Predict  whether 
in  practice  2.5  g.  of  each  of  these  salts  would  be  completely  or  only  partially 
decomposed  by  25  cc.  of  3  n.  Na2C03  solution. 

5.  Derive  the  mass-action  expression  (by  a  method  like  that  used  in 
Note  7,  P.  101)  for  the  equilibrium  that  would  result  when  a  slightly  soluble 
salt  with  univalent  anion  (like  AgCl)  is  treated  with  Na2C03  solution. 

6.  With  the  aid  of  the  result  obtained  in  answering  the  preceding  ques- 
tion and  the  solubility  values  involved,  calculate  the  chloride-ion  concen- 
tration that  would  result  on  treating  an  excess  of  AgCl  with  3  n.  Na2CO3 
solution,  taking  the  carbonate-ion  concentration  in  the  latter  as  2  normal 
(or  2000  millinormal).     Predict  whether  2.5  g.  of  AgCl  would  be  completely 
decomposed,  considerably  decomposed,  or  scarcely  at  all  acted  on  by  25  cc. 
of  3  n.  Na2C03  solution  at  20°. 

Experiments  34  and  35.  —  Detection  of  the  Chloride-Group.  —  i.  Arrange 
the  silver  salts  of  all  the  acidic  constituents  included  in  this  System  of 
Analysis  in  three  groups  comprising  respectively :  those  very  soluble  in 
water;  those  only  slightly  soluble  in  water,  but  readily  soluble  in  dilute 
HN03 ;  and  those  only  slightly  soluble  in  dilute  HN03. 

2.  Explain  by  reference  to  the  solubilities  of  the  salts  and  the  ioniza- 
tions  of  the  corresponding  acids  why  each  of  the  salts  in  the  second  group 
referred  to  in  the  previous  question  dissolves  in  dilute  HNO3. 

3.  Explain  why  the  silver  halides  do  not  dissolve  in  dilute  HNO3. 

4.  Explain  why  Ag2S  is  only  slightly  soluble  in  dilute  HNO3,  even 
though  the  ionization  of  HS~  is  extremely  small. 

5.  Explain  why  Ag2(CN)2  is  only  slightly  soluble  in  dilute  HNO3,  even 
though  HCN  is  very  slightly  ionized. 


34  QUESTIONS  ON  THE  EXPERIMENTS 

6.  If  the  NaNO2  reagent  contained  chloride  as  an  impurity,  how  could 
this  be  removed  in  a  simple  way  that  would  not  interfere  with  the  use  of 
the  reagent  for  this  test? 

Experiment  36.  —  Detection  of  the  Sulfate-Group.  —  i.  Arrange  the 
barium  salts  of  all  the  acidic  constituents  included  in  this  System  of  Analysis 
in  four  groups  comprising  respectively :  those  very  soluble  in  water ;  those 
only  slightly  soluble  in  water,  but  very  soluble  in  HAc ;  those  only  slightly 
soluble  in  HAc,  but  very  soluble  in  dilute  HC1;  and  those  only  slightly 
soluble  in  dilute  HC1. 

2.  What  is  the  purpose  of  adding  CaCl2  solution  with  the  BaCl2  solu- 
tion?   What  facts  m  regard  to  solubility  make  this  addition  necessary? 

3.  If  the  CaCl2  reagent  contained  sulfate  as  an  impurity,  how  could  this 
be  removed  in  a  simple  way  that  would  not  interfere  with  the  use  of  the 
reagent  for  this  test? 

Experiment  37.  —  Detection  of  Oxidizing  and  Reducing  Constituents.  — 
i.  Write  the  chemical  formulas  of  all  the  oxidizing  acids  which  produce  a 
dark  color  with  the  MnCl2  reagent,  and  of  the  product  to  which  each  of 
these  acids  might  be  expected  to  be  reduced  by  the  reagent. 

2.  With  the  aid  of  the  method  described  in  Note  10,  P.  21,  write  chemical 
equations  expressing  the  action  of  each  of  these  acids  on  MnCl2. 

3.  Explain  how  the  presence  of  sulfide  (or  other  reducing  constituent) 
in  the  substance  might  prevent  the  simultaneous  presence  of  nitrate  (or 
other  oxidizing  constituent)  from  being  detected  by  the  MnCl2  test.     State 
what  relative  molecular  quantities  of  sulfide  and  nitrate  would  cause  the 
MnCl2  test  to  yield  a  positive  result,  and  what  would  cause  it  to  yield  a 
negative  result. 

4.  Write  chemical  equations  expressing  the  action  of  each  one  of  the 
reducing  acids  on  K3Fe(CN)6,  and  that  of  H2SO3  on  Fe(NO3)3. 

Experiment  38.  —  Identification  of  Acidic  Constituents  by  the  Group- 
Reagents.  —  i.  Make  a  table  showing,  opposite  the  name  of  each  one  of  the 
acidic  constituents,  its  behavior  toward  each  one  of  the  four  group-reagents. 

2.  Name  all  the  acidic  constituents  that  can  be  present  in  each  of  the 
f ollowing  cases :   (a)  none  of  the  four  group-reagents  gives  a  positive  result ; 
(V)  the  chloride-group  reagent  yields  a  precipitate,  the  sulf  ate-group  reagent 
yields  no  precipitate,  and  oxidizing  and  reducing  constituents  are  both 
found  absent ;  (c)  the  chloride-group  reagent  yields  no  precipitate,  the  sul- 
fate-group  reagent  produces  a  precipitate,  oxidizing  constituents  are  found 
absent,  and  reducing  constituents  are  found  present. 

3.  A  solution  known  to  contain  only  a  single  constituent  gives  a  yellow 
chloride-group  precipitate,  a  blue  precipitate  with  the  ferricyanide  reagent, 
and  negative  results  with  the  other  two  reagents.    What  constituent  is 
present  ? 


QUESTIONS  ON  THE  EXPERIMENTS  35 

4.  A  solution  known  to  contain  only  a  single  constituent  gives  a  brown 
color  with  the  MnCl2  reagent,  but  negative  results  with  the  other  three 
reagents.    What  constituent  is  present? 

5.  Explain  how  nitrite  can  act  both  as  an  oxidizing  and  a  reducing 
constituent. 

Experiment  39.  —  Analysis  of  the  Chloride-Group.  —  i.  With  the  aid  of 
the  Table  of  Solubilities  in  the  Appendix  predict  what  constituents  beside 
sulfide  might  be  precipitated  on  the  addition  of  Pb(NO3)2  to  the  Na2C03 
solution,  wholly  or  in  part.  Explain  whether  this  is  determined  solely  by 
the  relative  solubilities  of  PbC03  and  the  other  lead  salts  in  water. 

2.  Explain  whether  the  Pb(NO3)2  could  be  replaced  by  AgNO3. 

3.  Explain  the  fact  that  no  HCN  is  set  free  on  acidifying  solutions  of 
Na3Fe(CN)6  and  Na4Fe(CN)6. 

4.  Suggest  possible  explanations  of  the  fact  that  Ni(CN)2  is  only  slightly 
soluble  in  HAc. 

5.  The  reducing  constituents  tested  for  hi  P.  105  all  reduce  HC1O3  in 
a  strongly  acid  solution.    Of  these  constituents  what  ones  might  be  used  hi 
place  of  HNO2  in  testing  for  chlorate  in  P.  108? 

Experiment  41.  —  Detection  of  Thiocyanate  and  the  Different  Halides.  — 
i.  Derive,  hi  the  way  described  in  Note  3,  P.  no,  the  relation  stated  in 
that  Note  to  exist  between  the  solubilities  in  water  and  those  in  dilute 
NH4OH  of  slightly  soluble  silver  salts. 

2.  A  saturated  solution  of  AgCl  hi  2  n.  NH4OH  at  25°  is  0.15  normal. 
With  the  aid  of  the  principle  considered  in  the  preceding  question,  cal- 
culate approximately  how  many  milligrams  of  iodine  as  Agl,  and  of  bromine 
as  AgBr,  would  be  dissolved  by  that  quantity  of  2  n.  NHjOH  required  to 
dissolve  210  mg.  of  chlorine  as  AgCl.     State  whether  a  good  analytical 
separation  of  any  two  of  these  three  acidic  constituents  could  be  based  on 
the  difference  in  the  solubilities  of  the  silver  salts  hi  NH4OH. 

3.  With  the  aid  of  the  values  of  the  specific  reduction-potentials  given 
in  the  Table  in  the  Appendix  explain  why  Fe(N03)3  liberates  I2  from  an 
iodide  almost  completely,  but  sets  free  scarcely  any  Br2  from  a  bromide. 

4.  Predict  whether  any  reaction  would  occur,  and  whether  or  not  it 
would  be  practically  complete,  on  mixing  the  following  substances  (the 
halogen  being  in  moderate  excess) :    (a)  Br2  and  KC1 ;   (b)  Br2  and  KI ; 
(c)  C12  and  KBr ;  (d)  C12  and  FeCl2 ;   (e)  I2  and  FeI2. 

5.  If  loo  mg.  of  I2  were  present  in  10  cc.  of  solution,  what  quantity  of  it 
would  remain  in  the  aqueous  layer  after  it  was  shaken  with  i  cc.  of  CCU? 
After  it  was  shaken  a  second  time  with  i  cc.  of  CCLj?    After  it  was  shaken  a 
third  time  with  i  cc.  of  CCU?    What  quantity  would  remain  hi  the  aqueous 
layer  if  it  were  shaken  at  first  with  3  cc.  of  CCLj? 

6.  If  100  mg.  of  Br2  were  present  in  10  cc.  of  solution,  what  quantity  of 
it  would  remain  hi  the  aqueous  layer  after  it  was  shaken  with  i  cc. 


36  QUESTIONS  ON  THE  EXPERIMENTS 

7.  Explain  why  the  time  required  for  liberating  the  bromine  from  a 
bromide  by  KMnO4  would  be  diminished  much  more  by  doubling  the  con- 
centration of  the  HN03  than  by  doubling  that  of  the  KMnO4. 

8.  Write  the  equation  expressing  the  formation  of  Mn02  by  the  action 
of  KMnO4  on  HBr;  also  the  equation  expressing  the  action  of  HN02  on 
MnO2  in  HNO3  solution. 

9.  What  error  might  result  if  the  mixture  were  not  boiled  long  enough 
before  adding  the  NaN02  and  AgN03?     What  test  might  be  made  before 
adding  these  reagents,  to  guard  against  this  error  ? 

-  Experiment  44.  —  Analysis  of  the  Sul 'fate-Group.  —  i.  Explain  with  the 
aid  of  the  solubilities  of  the  salts  involved  whether  sulfide  could  be  removed 
in  this  case,  as  in  P.  106,  by  adding  Pb(NO3)2  to  the  Na2C03  solution  and 
filtering  the  mixture  before  acidifying  it. 

2.  Why  must  sulfide  and  thiocyanate  be  removed  before  testing  for 
sulfite? 

3.  By  considering  the  ionization  and  solubility  values  involved,  explain 
why  BaS03  and  BaCrO4  are  not  precipitated  in  the  presence -of  HC1,  but  are 
so  precipitated  when  only  HAc  is  present ;  and  why  BaHPO4  is  not  pre- 
cipitated even  in  the  presence  of  HAc. 

4.  State  and  explain  with  reference  to  the  solubility  and  ionization 
values  involved  how  fluoride  behaves  in  each  step  of  P.  in. 

5.  Make  a  similar  statement  and  explanation  as  to  how  oxalate  would 
behave  in  P.  in. 

6.  Suggest  a  reason  why  chromate,  though  it  is  one  of  the  constituents 
of  the  sulfate-group,  is  not  included  in  the  mixture  (of  Na2S,  Na2SO4,  Na2SO3, 
and  NaF)  used  in  illustrating  the  method  of  analysis  of  that  group. 

7.  Write  the  chemical  equations  expressing  all  the  successive  reactions 
involved  in  treating  CaF2  by  the  confirmatory  test  for  fluoride  (P.  112). 

Experiment  45.  —  Detection  of  Nitrate  or  Nitrite.  —  i.  Write  the  chemi- 
cal equations  expressing  the  action  of  Al  on  NaOH  solution,  and  that  of  the 
hydrogen  thereby  produced  on  NaN02  and  NaN03. 

2.  State  why  cyanides  and  thiocyanate  must  be  removed  before  carrying 
out  this  Procedure;  and  explain  with  reference  to  the  solubilities  of  the 
silver  salts  involved  why  they  can  be  removed  by  adding  Ag2C03  to  the 
Na2CO3  solution. 

Experiment  50.  —  Detection  of  Sulfate,  Carbonate,  Sulfide,  and  Cyanide.  — 
i.  Describe  how  sulfite  would  interfere  with  the  test  for  carbonate  in  P.  117. 
State  how  a  precipitate  of  BaS03  would  be  distinguished  from  one  of  BaCO3. 
Explain  how  the  addition  of  H2O2,  directed  in  Note  2,  P.  117,  removes  the 
difficulty  of  detecting  carbonate  in  the  presence  of  sulfite. 

2.  Write  chemical  equations  expressing  the  action  of  Zn  and  HC1  on 
CuS,  As2S3,  FeS2,  and  PbS04. 


QUESTIONS  ON  THE  EXPERIMENTS  37 

3.  Write  the  chemical  equations  expressing  all  the  successive  reactions 
in  the  test  for  cyanide  in  P.  121. 

Experiment  51.  —  Detection  of  Chloride,  Fluoride,  and  Bar  ate.  —  i.  State 
what  influence  the  presence  of  much  silica  in  the  substance  would  have  on 
the  detection  of  fluoride  in  P.  122. 

2.  By  considering  the  ionization-values  involved,  state  to  what  extent 
HBO2  is  displaced  from  borates  by  H2SO4 ;  and  explain  why  it  does  not  pass 
over  into  the  distillate  till  CH3OH  is  added. 


PREPARATION  OF   THE   SOLUTION   FOR    THE    DETECTION  OF    BASIC 

CONSTITUENTS    AND    THE    COMPLETE    ANALYSIS    OF 

UNKNOWN   SUBSTANCES 

Experiment  52.  —  Indications  of  Certain  Constituents  Afforded  by  the 
Closed-Tube  Test.  —  i.  Of  what  elements  are  organic  substances  (carbon- 
containing  compounds)  most  commonly  composed,  and  what  causes  them 
to  blacken  on  heating? 

2.  Explain  whether  the  presence  of  a  large  quantity  of  sugar  in  a  sub- 
stance would  interfere  with  the  precipitation  of  the  copper-  and  tin-groups, 
with  that  of  the  aluminum-  and  iron-groups,  and  with  that  of  the  alkaline- 
earth  group ;  also  whether  its  presence  would  interfere  with  the  detection 
of  the  alkali-elements. 

3.  Name  the  different  forms  in  which  water  may  be  present  in  a  sub- 
stance. 

4.  If  a  substance  were  ignited  at  a  red  heat  (for  example,  to  destroy 
organic  matter)  before  submitting  it  to  analysis,  what  basic  constituents 
would  be  lost  ? 

5.  State  how  each  of  the  following  substances  would  behave  in  the 
closed-tube  test,  and  write  the  chemical  equation  expressing  the  reaction 
that  takes  place:   MgNH4PO4,  KHS04,  PbC03,  KC1O3,  As2O3. 

Experiment  53.  —  Analysis  of  Non-Metallic  Substances  Dissolved  by 
Water  or  Dilute  Acid.  —  i.  Tabulate  the  behavior  shown  by  each  of  the 
following  substances  when  treated  with  water,  tested  with  litmus,  and 
treated  with  2  n.  HNO3,  cold  and  hot,  as  directed  hi  P.  2 :  Bi(NO3)3,  FeC03, 
Na2SiO3,  KI,  CdS,  KCN,  KAg(CN)2,  Ca3(PO4)2,  BaSO3,  SnCl2. 

2.  What  determines  whether  a  sodium  salt  is  hydrolyzed  so  as  to  give 
an  alkaline  reaction  to  litmus,  and  what  determines  whether  a  nitrate  is 
hydrolyzed  so  as  to  give  an  acid  reaction  to  litmus  ? 


38  QUESTIONS  ON  THE  EXPERIMENTS 

Experiment  54.  —  Analysis  of  Non-Metallic  Substances  Dissolved  only 
by  Concentrated  Acids.  —  i.  What  acidic  constituent  forms  salts  many  of 
which  are  not  dissolved  by  cold  dilute  HNO3,  but  are  decomposed  by  hot 
concentrated  HNO3  because  of  its  oxidizing  action  ? 

2.  State  what  happens  (that  is,  what  chemical  changes  occur  and  what 
phenomena  are  observed)  at  each  step  of  the  process  when  each  of  the 
following  substances  is  treated  by  P.  3 :   CuSiO3  (a  silicate  decomposed  by 
acids),  Sb2S3,  Mn02,  PbCrO4,  PbS04,  Ag3PO4,  HgS,  Hg2Cl2,  Fe304. 

3.  Give  examples  of  substances  which  dissolve  in  the  acids  used  in  P.  3 
in  virtue  (a)  of  the  action  of  the  acids  as  such ;  (b)  of  the  oxidizing  action, 
and  (c)  of  the  reducing  action. 

Experiment  55.  —  Analysis  of  Alloys.  —  i.  What  elements  are  scarcely 
ever  found  in  alloys,  and  how  in  consequence  may  the  analysis  of  an  alloy 
be  shortened? 

2.  State  the  result  of  treating,  as  in  P.  4,  each  of  the  following  alloys, 
(a)  with  6  n.  HNO3,  and  (b)  with  6  n.  HC1  (after  adding  HC1  and  evaporat- 
ing the  mixture) :  brass  (Cu,  Zn) ;  solder  (Pb,  Sn) ;  ferrosilicon  (Fe,  Si,  C) ; 
corn-silver  (Ag,  Cu). 

Experiment  56.  —  Analysis  of  Natural  Substances  or  Igneous  Products 
Not  Completely  Dissolved  by  Treatment  with  Nitric  and  Hydrochloric  A  cids.  — 
i.  State  what  happens  to  each  of  the  substances  whose  symbols  are  given 
under  (b)  hi  Table  I  when  it  is  treated  as  in  P.  5 :  (a)  with  concentrated 
H2S04 ;  (6)  then  with  HF ;  (c)  then  (after  evaporating)  with  dilute  H2SO4. 
State  also  what  happens  on  treating  with  Na2CO3  solution  by  P.  6  any 
residue  undissolved  by  the  dilute  H2SO4. 

2.  State  what  happens  to  feldspar  (potassium  aluminum  silicate,  an 
example  of  a  silicate  not  much  decomposed  by  acids  other  than  HF)  when 
it  is  treated  by  P.  5. 

3.  State  what  happens  to  each  of  the  substances  whose  symbols  are 
given  under  (6)  in  Table  I  (including  also  feldspar  as  an  example  of  a  silicate) 
on  fusing  it  with  Na2CO3  and  NaNO3,  on  treating  the  fused  mass  with  water, 
and  on  treating  the  residue  with  dilute  HNO3,  as  described  hi  P.  7. 

4.  State  on  which  of  these  substances  the  NaNO3  added  in  the  fusion 
has  an  effect,  and  what  that  effect  is  in  each  case. 


PART   II 
THE   SYSTEM    OF  ANALYSIS 


PREPARATION  OF  THE  SOLUTION 

FOR  THE 

DETECTION  OF  THE  BASIC  CONSTITUENTS 


GENERAL  DIRECTIONS 

Procedure  i. — Preliminary  Examination  and  General  Di- 
rections. —  In  case  the  substance  is  a  non-metallic  solid,  note  its 
color,  odor,  and  texture;  examine  it  with  a  lens  to  determine 
whether  it  is  heterogeneous,  and,  if  so,  note  the  appearance 
of  its  constituents.  To  determine  whether  organic  matter  or 
water  is  present  and  to  get  other  indications,  heat  gently  at  first, 
then  strongly,  about  o.i  cc.  of  the  finely  powdered  substance 
in  a  hard-glass  tube  (of  about  10  mm.  bore  and  100  mm.  length) 
closed  at  one  end.  Note  whether  the  substance  blackens, 
whether  a  tarry,  aqueous,  or  other  deposit  forms  on  the  cold 
part  of  the  tube,  and  whether  any  odor  is  emitted. 

If  organic  matter  is  thus  proved  absent,  prepare  a  solution  of 
the  substance  by  treating  a  sample  of  it  by  P.  2-3  and  P.  5-6 
(or  7).  If  organic  matter  is  proved  present,  treat  the  substance 
by  P.  8.  Treat  the  so-prepared  solution  or  solutions  as  directed 
in  these  Procedures,  to  detect  basic  constituents. 

Treat  fresh  samples  of  the  substance  by  P.  91  to  detect  am- 
monium, and  by  P.  92,  if  necessary,  to  determine  the  state  of 
oxidation  of  certain  basic  constituents. 

Treat  fresh  samples  of  the  substance  as  directed  in  P.  100, 
to  detect  acidic  constituents. 

39 


40  PREPARATION  OF  THE  SOLUTION  P.  1 

In  case  the  substance  is  an  alloy,  prepare  a  solution  of  it  by 
P.  4,  and  analyze  this  solution  by  P.  21-79.  If  there  is  a  residue, 
treat  it  by  P.  5,  followed  by  P.  n-68. 

In  case  the  substance  is  a  solution,  treat  it  as  directed  in  P.  9. 

Notes.  —  i.  When  a  complete  analysis  in  the  wet  way  is  to  be  made, 
it  is  usually  not  worth  while  to  make  a  more  extended  preliminary 
examination  in  the  dry  way.  The  closed-tube  test  is,  however,  essential, 
in  order  to  show  whether  organic  matter  is  present ;  for  certain  kinds 
of  organic  matter,  especially  sugars  and  hydroxy-acids,  such  as  tartaric, 
citric,  and  lactic  acids,  prevent  the  precipitation  of  the  hydroxides  of 
most  of  the  elements  by  alkalies.  Such  organic  matter  must  therefore 
be  detected  and  removed  in  order  to  insure  the  precipitation  of  alumi- 
num and  chromium  by  NH4OH.  Moreover,  a  large  quantity  of  organic 
matter  of  any  kind  interferes  with  the  execution  of  the  analysis;  for 
example,  with  the  operations  of  solution,  filtration,  and  evaporation. 
Alloys  do  not  contain  organic  matter  or  water;  therefore  the  closed- 
tube  test  need  not  be  applied  to  them. 

2.  Blackening  accompanied  by  a  burnt  odor  or  by  the  formation 
of  a  tarry  deposit  shows  organic  matter.     Blackening  alone  does  not 
show  it ;  for  copper,  cobalt,  and  nickel  salts  may  turn  black  on  heating, 
owing  to  the  formation  of  the  black  oxides.     Oxalates  blacken  and 
emit  a  burnt  odor  to  a  much  less  extent  than  other  organic  substances, 
and  may  not  be  detected  by  the  closed-tube  test. 

3.  It  is  usually  desirable  to  determine  whether  water  is  a  constituent 
of  the  substance,  and,  if  so,  whether  it  is  present  in  large  or  small  pro- 
portion.   This  can  be  done  with  a  fair  degree  of  delicacy  by  the  closed- 
tube  test,  provided  care  be  taken  to  keep  the  upper  part  of  the  tube  cool 
during  the  first  of  the  heating.    Water  may  be  present  as  so-called 
water  of  constitution,  as  in  Fe03H3  or  Na2HP04 ;  as  water  of  crystalliza- 
tion, as  in  MgSCX  •  7  H20 ;    as  inclosed  water,  as  in  some  hydrated 
silicates  like  the  zeolites  or  as  mother-liquor  within  crystals;   and  as 
hygroscopic  moisture  on  the  surface.     Water  of  constitution  may  be 
expelled  only  at  a  fairly  high  temperature,  while  in  the  other  forms  it 
is  seldom  retained  above  200°. 

4.  The  closed-tube  test  may  also  furnish  evidence  of  the  presence  of 
certain  basic  and  acidic  constituents  when  they  are  present  in  consider- 
able quantity.    Thus  all  ammonium  salts  and  mercury  compounds  are 
volatilized  much  below  a  red  heat.    Ammonium  salts  and  the  chlorides 
of  mercury  give  a  white  sublimate.     Most  other  mercury  compounds 
give  a  gray  one,  consisting  of  minute  globules  of  mercury,  made  visible 


P.  /  PREPARATION  OF  THE  SOLUTION  41 


by  a  lens  or  by  rubbing  with  a  wire.  Metallic  As,  As^Os,  and  AsjSa  are 
also  readily  volatilized,  forming  black,  white,  and  yellow  sublimates, 
respectively.  Of  the  acid-forming  elements  or  groups,  free  sulfur  or  a 
persulfide  is  shown  by  a  sublimate  of  reddish-brown  drops,  changing  to 
a  yellow  solid  on  cooling,  and  accompanied  by  odor  of  SO2  ;  a  moist 
sulfide,  by  the  odor  of  H2S  ;  a  nitrate  or  nitrite,  by  brcwn  vapors  of 
NO2  ;  free  iodine  or  a  decomposable  iodide,  by  a  black  sublimate  of 
I2  and  by  its  violet  vapor  ;  a  sulfite,  by  the  odor  of  S02  ;  a  peroxide, 
chlorate,  or  nitrate,  by  evolution  of  oxygen,  recognized  by  its  inflaming 
a  glowing  wood-splinter  held  in  the  tube  ;  and  a  carbonate  or  oxalate, 
by  the  evolution  of  C02,  recognized  by  its  causing  turbidity  in  a  drop 
of  Ba(OH)2  solution. 


PREPARATION  OF  THE  SOLUTION 


P.  2 


PREPARATION   OF   THE   SOLUTION   FOR   THE   DETECTION    OF    BASIC 

CONSTITUENTS 

TABLE  I.  —  PREPARATION  OF  THE  SOLUTION  IN  THE   CASE   OF    NON- 
METALLIC  SOLID  SUBSTANCES  FREE  FROM  ORGANIC  MATTER 


Heat  the  substance  with  water  and  dilute  HN03  (P.  2). 


It  all  dis- 
solves. 
Treat  the 
solution 
by  P.  ii. 


B.  It  does  not  all  dissolve. 

Residue  :*  (a)  Sb206)  H2Sn03,  Fe2O3,  MnO2)  Pb02,  S,  many 
sulfides,  BaCr04,PbCr04.  (&)  C,  SiC,  A12O3)  Cr2OS)  AgCl, 
CaF2)  CaS04,  PbS04)  BaS04,  SrSO4,  SnS2)  Sn02,  SiO2,  and 
many  silicates. 

Solution :  most  substances  as  nitrates. 

Without  filtering,  evaporate  the  mixture  to  2  cc.,  add  HCl, 
evaporate  completely,  heat  with  dilute  HCl,  and  filter  the  hot 
mixture  (P.  j). 


Solution  : 
Treat  by 

P.  21. 

Residue  :  substances  under  (ft)  . 
Heat  with  F2504  and  HF,  evaporate,  add  di- 
lute H»SOi,  boil  (P.  5). 

Gas: 

SiF4. 

Residue  : 
Pb,  Ba,  Sr,  Ca,  Cr, 

as  sulfates. 
Treat  by  P.  6. 

Solution  : 
Other  elements 
as  sulfates. 
Treat  by  P.  u. 

*  Only  the  more  common  substances  that  are  likely  to  be  present  in  the  residue  are 
here  mentioned ;  and  some  of  these  may  pass  wholly  or  partially  into  the  solution. 

Procedure  2.  —  Treatment  of  Non-Metallic  Substances  Free  from 
Organic  Matter.  —  Weigh  out  on  a  rough  balance  i  g.  of  the  finely 
powdered  substance  (see  Note  i),  add  to  it  in  a  conical  flask 
10  cc.  of  water,  heat  the  mixture  to  boiling  if  there  is  a  residue, 
and  test  the  solution  with  litmus  paper.  Add  to  the  mixture, 
if  it  is  not  already  acid,  6  n.  HNOs,  a  few  drops  at  a  time,  till, 
after  shaking,  it  becomes  distinctly  acid.  Note  whether  there 
is  an  odor  or  effervescence.  Then  cool  the  mixture,  and  add  to 
it,  without  filtering  out  any  residue,  just  5  cc.  of  6  n.  HNOa. 
If  there  is  a  residue  (but  not  otherwise),  heat  the  mixture  nearly 
to  boiling  for  2  or  3  minutes,  covering  the  flask  with  a  watch- 
glass  and  not  letting  the  mixture  actually  boil. 


P.  2  PREPARATION  OF  THE  SOLUTION  43 

In  case  the  substance  has  dissolved  completely,  treat  the  solu- 
tion by  P.  ii. 

In  case  the  substance  has  not  dissolved  completely,  treat  the 
mixture,  without  filtering  out  the  residue,  by  P.  3. 

Notes.  —  i.  In  order  that  difficultly  soluble  substances  may  be 
dissolved,  the  substance  must  be  reduced  to  a  very  fine  powder.  This 
is  usually  best  accomplished  by  grinding  the  substance,  a  small  quantity 
at  a  time,  in  a  porcelain  or  agate  mortar.  With  hard  substances,  and 
in  general  with  minerals,  an  agate  mortar  should  be  used.  As  such  a 
mortar  is  likely  to  be  broken  by  a  blow,  the  substance  should  be  ground, 
not  pounded,  in  it. 

2.  The  quantity  of  the  substance  taken  for  analysis  should  always  be 
approximately  known ;  for  a  good  qualitative  analysis  should  not  only 
show  the  presence  or  absence  of  the  various  elements  in  the  substance, 
but  should  enable  their  relative  quantities  to  be  estimated.    Since 
i  or  2  mg.  of  almost  any  element  can  be  detected  by  this  system  of 
analysis,  the  presence  of  0.1-0.2%  of  an  element  will  be  detected  when 
one  gram  of  substance  is  taken,  and  this  degree  of  delicacy  is  ordinarily 
sufficient.    If  much  more  than  this  quantity  is  taken,  the  precipitates 
may  be  so  large  that  much  time  is  consumed  in  filtering  and  washing 
them.     Moreover,  the  directions  given  for  many  of  the  separations  are 
based  on  the  assumption  that  not  more  than  500  mg.  of  any  one  con- 
stituent is  present. 

3.  The  substance  is  treated  with  only  10  cc.  of  water  so  that  the 
HN03  subsequently  added  may  be  concentrated  enough  to  prevent  the 
hydrolysis  of  salts  of  bismuth,  antimony,  and  tin,  and  thus  insure  their 
solution.     The  mixture  is  cooled  before  the  addition  of  the  main  quan- 
tity of  HNOs  so  as  to  avoid  oxidizing  mercurous,  arsenous,  and  ferrous 
salts  unnecessarily.     But,  in  case  there  is  a  residue,  the  mixture  is 
heated,  since  the  hot  acid,  largely  owing  to  its  oxidizing  action,  has  a 
greater  solvent  effect  on  many  substances,  notably  on  sulfides. 

4.  To  what  extent  the  substance  dissolves  in  water  and  in  dilute 
HNOs  should  be  noted,  since  it  furnishes  important  indications  of  the 
nature  of  the  constituents  present.     General  statements  as  to  the 
solubilities  of  chemical  substances  in  water  and  dilute  acid  will  be 
found  in  the  Appendix  under  "  Solubilities."    When,   however,   the 
substance  to  be  analyzed  dissolves  only  partly  in  the  water  or  in  the 
dilute  HNO3,  it  is  usually  not  worth  while  to  filter  off  the  residue  and 
analyze  it  and  the  solution  separately.    This  need  be  done  only  when 
special  information  is  desired  as  to  the  water-soluble  and  acid-soluble 
constituents, 


44  PREPARATION  OF  THE  SOLUTION  P.  2 

5.  The  residue  undissolved  by  HNO3  probably  consists  of  one  or 
more  of  the  substances  whose  formulas  are  given  in  Table  I  under  (a) 
and  (6).     Some  of  these  substances  (for  example,  Fe2O3and  A12O3)  are 
really  soluble  in  the  acid ;  but,  when  in  the  form  of  native  or  ignited 
products,  they  may  fail  to  dissolve  because  of  a  very  slow  rate  of 
solution.     Other  less  common  substances  that  may  be  present  in  the 
residue  are  anhydrous  chromium  salts,  stannic  phosphate,  and  the 
ferrocyanides  of  iron  and  of  some  other  elements. 

6.  Just  5  cc.  of  6  n.  HN03  are  added  at  this  point,  in  order  that 
the  acid  concentration  may  be  properly  adjusted  in  the  subsequent 
H2S  precipitation.     For  this  reason,  when  the  solution  is  alkaline  or 
when  a  substance  (like  an  undissolved  oxide  or  carbonate)  which 
neutralizes  the  acid  is  present,  the  solution  is  made  distinctly  acid 
before  adding  the  5  cc.  of  HNO3.     For  the  same  reason  care  is  taken 
to  prevent  loss  of  the  acid  by  evaporation. 

7.  If  the  aqueous  solution  has  an  alkaline  reaction,  the  addition  of 
an  acid  may  cause  precipitation  of  any  substance  held  in  solution  by 
an  alkaline  solvent;   for  example,  sulfur  or  sulfides  of  the  tin-group 
from  an  alkaline  sulfide  solution ;    silver  chloride  or  cyanide  from  a 
potassium  cyanide  solution ;  silicic  acid  from  sodium  silicate  solution ; 
or  basic  hydroxides  from  solutions  in  alkalies.    These  last  substances 
redissolve  when  the  excess  of  HNO3  is  added. 

8.  An  acid  reaction  of  the  aqueous  solution  towards  litmus  is  due 
to  hydrogen-ion,  which  may  arise  from  free  acid,  from  an  acid  salt  of 
a  strong  acid,  or  (by  hydrolysis)  from  a  neutral  salt  of  a  strong  acid 
and  a  weak  base.     An  alkaline  reaction  is  due  to  hydroxide-ion,  which 
may  arise  from  a  soluble  hydroxide,  or  (by  hydrolysis)  from  a  carbonate, 
sulfide,  phosphate,  borate,  cyanide,  or  a  salt  of  some  other  weak  acid. 

9.  When  the  acid  is  added  to  the  aqueous  solution,  the  evolution  of 
any  gas  and  its  odor  should  be  noted,  since  this  indicates  the  nature 
of  the  acidic  constituents  present.    Thus  carbonates  effervesce  with 
evolution  of  CO2;    sulfides  produce  the  odor  of  H2S;    sulfites  and 
thiosulf ates,  that  of  SO2 ;  and  cyanides,  that  of  HCN. 

10.  On  heating  the  HNO3  solution,  sulfur  may  separate  as  a  spongy 
or  pasty  mass,  indicating  the  presence  of  sulfide ;  iodine  may  be  liberated 
from  an  iodide,  producing  a  black  precipitate,  a  brown  color  in  the 
solution,  or  violet  vapors  above  it ;  bromine  may  be  set  free  from  a 
bromide,  yielding  a  red  solution ;  nitrogen  peroxide  may  be  produced 
by  action  of  the  HNO3  on  a  sulfide,  sulfite,  or  iodide,  or  on  a  mercurous, 
stannous,  or  ferrous  compound ;  silicic  acid  may  be  set  free  as  a  gelat- 
inous precipitate,  indicating  the  presence  of  a  decomposable  silicate ; 
and  a  white  amorphous  precipitate  of  antimonic  oxide  (Sb206)  or 
metastannic  acid  (H2SnO3)  may  separate, 


P.  3  PREPARATION  OF  THE  SOLUTION  45 

Procedure  3.  —  Further  Treatment  of  Non-Metallic  Substances 
Not  Dissolved  by  Dilute  Nitric  Acid.  —  In  case  the  substance 
has  not  dissolved  in  dilute  HN03,  transfer  the  unfiltered  mixture 
(P.  2)  to  a  casserole,  evaporate  it  to  about  2  cc.,  add  5  cc.  of 
12  n.  HC1,  and  evaporate  slowly  just  to  dryness. 

Heat  the  residue  carefully  at  100-130°  till  it  is  dry,  keep- 
ing the  casserole  in  motion  over  a  small  flame.  Loosen  the 
residue  from  the  dish,  and  rub  it  to  a  fine  powder  with  a  pestle ; 
add  to  it  just  5  cc.  of  6  n.  HC1,  cover  the  dish,  and  warm  the 
mixture,  taking  care  that  none  of  the  acid  evaporates.  Add 
10  cc.  of  water  to  the  mixture,  and  heat  it  just  to  boiling.  (If 
there  is  a  residue  that  seems  to  be  gradually  dissolving,  add 
2  cc.  of  12  n.  HC1,  evaporate  the  mixture  slowly  almost  to  dry- 
ness,  and  heat  the  moist  residue  with  5  cc.  of  6  n.  HC1  and 
10  cc.  of  water,  as  before.)  Filter  the  boiling-hot  mixture. 
Treat  the  filtrate  by  P.  21.  Wash  the  residue  with  5-10  cc. 
of  2  n.  HC1  and  then  thoroughly  with  hot  water  (rejecting  all 
the  washings),  and  treat  it  by  P.  5  (or  by  P.  7  in  case  the  use 
of  a  platinum  crucible  or  of  hydrofluoric  acid  is  impracticable) . 

Notes.  —  i.  The  concentrating  of  the  HNO3  solution  and  the 
subsequent  addition  of  HC1  to  it  produce  a  strongly  oxidizing  solvent, 
which  dissolves  all  sulfides  (except  ignited  SnS2).  The  mixture  of 
HNOa  and  HC1,  which  is  known  as  aqua  regia,  owes  its  powerful  oxidiz- 
ing action  to  the  fact  that  these  acids  react  with  each  other,  when 
warm  and  concentrated,  with  the  formation  of  C12  and  NOC1  (nitrosyl 
chloride) . 

2.  Enough  HC1  is  added  to  destroy  finally  all  the  HNO3.    The 
concentrated  HC1  remaining  then  exercises  a  reducing  action  on  such 
substances  as  MnO2,  PbO2,  PbCrO4,  and  BaCr04,  whereby  they  are 
converted  into  soluble  compounds. 

3.  The  hot  concentrated  HC1  acts,  moreover,  as  a  powerful  acid 
solvent  on  slowly  dissolving  oxides,  such  as  SbaOs,  SnO2,  Fe5O3,  A1203. 
Its  action  in  this  respect  is  far  more  rapid  and  effective  than  that  of 
HN03.    To  allow  time  for  it  to  act,  it  is  directed  that  the  acid  be 
evaporated  slowly. 

4.  It  will  be  seen  from  the  foregoing  statements  that,  by  using  the 
two  acids  in  the  way  directed  in  the  Procedure,  three  types  of  solvent 
action  are  secured.    The  important  substances  which  may  resist  this 


46  PREPARATION  OF  THE  SOLUTION  P.  8 

treatment  are  those  whose  formulas  are  given  in  Table  i  under  (ft).  Of 
these  substances  CaSO4  and  CaF2  dissolve  in  considerable  quantity, 
and  SrSO4  and  PbSO4  in^mall  quantity,  in  the  dilute  HC1.  The  slightly 
soluble  sulfates  may  not  have  been  present  in  the  original  substance, 
but  may  have  been  produced  by  oxidation  from  some  sulfide  when 
the  corresponding  basic  elements  are  also  present. 

5.  Provided  the  substance  has  been  treated  by  this  Procedure,  all 
the  silver  originally  present,  whatever  may  have  been  its  form,  will 
be  left  in  the  residue  as  AgCl.     Any  mercury,  arsenic,  antimony,  tin, 
or  iron  present  in  the  solution  will  be  in  the  higher  state  of  oxidation ; 
any  chromium  will  be  in  the  form  of  chromic  chloride,  and  any  manga- 
nese in  the  form  of  manganous  chloride. 

6.  When  a  silicate  is  decomposed  by  acid,  silicic  acid  may  separate 
as  a  gelatinous  precipitate,  but  even  then  a  part  of  it  always  remains 
in  solution,  mainly  as  a  colloid.    When  thoroughly  dried  at  100-130°, 
it  is  partially  dehydrated  and  becomes  entirely  insoluble  in  acid.    The 
HC1  solution  is  therefore  evaporated  to  dryness  and  the  residue  is 
heated  at  100-130°,  in  order  to  remove  the  silica  at  this  point ;  for,  if 
it  were  not  removed,  it  would  appear  as  a  gelatinous  precipitate  at 
some  later  stage  of  the  analysis;   thus,  if  it  did  not  separate  earlier, 
it  would  be  precipitated  by  NH4OH  with  the  aluminum  and  iron  groups, 
and  might  then  be  mistaken  for  A1(OH)3.     Care  is  taken  to  avoid 
overheating,  since  it  may  cause  other  substances  to  dissolve  only  very 
slowly  in  dilute  acid  and  may  cause  volatilization  of  mercury  and  tin. 
In  the  case  of  substances  which  cannot  contain  silica,  the  heating  may 
be  omitted. 

7.  The  foregoing  statements  show  that,  when  no  residue  is  left  un- 
dissolved  by  the  dilute  HC1,  silver,  silica,  and  silicate  can  be  pronounced 
absent  in  the  substance;  but  that  this  is  not  true  of  any  other  con- 
stituent. 

8.  The  concentrated  HC1  is  completely  removed  by  evaporation, 
the  dried  residue  is  treated  with  just  5  cc.  of  6  n.  HC1,  care  is  taken 
to  prevent  evaporation  of  the  acid,  and  the  acid  washings  are  not 
collected  with  the  filtrate,  so  as  to  enable  the  acid  concentration  to  be 
properly  adjusted  in  the  subsequent  precipitation  with  H2S. 

9.  Only  10  cc.  of  water  are  added  to  the  HC1  solution  so  that  the 
acid  may  be  concentrated  enough  to  hold  in  solution  even  500  mg.  of 
bismuth  and  as  much  antimony  as  possible  (about  60  mg.).     The 
mixture  is  filtered  boiling-hot  so  that  lead  may  pass  into  the  nitrate 
(which  it  does  up  to  about  200  mg.).    The  residue  is  washed  first  with 
2  n.  HC1  to  remove  bismuth  and  antimony  salts,  and  then  with  hot 
water  to  remove  other  soluble  substances,  including  any  PbCl2  still 
present, 


P.  4  PREPARATION  OF  THE  SOLUTION  47 

Procedure  4.  —  Treatment  of  Alloys.  —  In  case  the  substance 
is  an  alloy,  convert  it  into  a  form  offering  a  large  surface  (see 
Note  i),  and  treat  0.5  g.  of  it  in  a  casserole  with  5  cc.  of  6  n. 
HN03.  Cover  the  dish  with  a  watch-glass,  and  heat  the  mix- 
ture nearly  to  boiling  so  long  as  any  action  continues,  adding 
i  cc.  of  1 6  n.  HNOs  if  any  of  the  alloy  is  still  unattacked ;  evapo- 
rate to  about  2  cc.,  add  5  cc.  of  12  n.  HC1,  and  evaporate  slowly 
just  to  dryness.  Treat  the  residue  as  directed  in  the  last  para- 
graph of  P.  3,  omitting  the  heating  at  100-130°  except  in  the 
case  of  iron  alloys. 

Notes.  —  i.  Many  alloys  cannot  be  powdered  by  grinding  in  a 
porcelain  or  agate  mortar.  They  may  usually  be  converted  into  a 
form  that  offers  a  large  surface  by  hammering  in  a  steel  mortar,  filing 
with  fine  steel  file,  shaving  with  a  knife,  or  converting  into  turnings 
with  a  lathe.  Only  0.5  g.  of  an  alloy  is  taken  for  analysis;  for,  owing 
to  the  absence  of  acidic  constituents,  the  same  quantity  of  basic  ele- 
ments is  contained  in  a  smaller  amount  of  substance. 

2.  Most  alloys  are  attacked  by  strong  HNO3,  all  the  elements  that 
may  be  present  going  into  solution,  except  antimony,  tin,  carbon,  and 
silicon.     Antimony  is  oxidized   to  antimonic  oxide   (SbjOs),   tin  to 
metastannic  acid  (wH2SnO3),  and  silicon  wholly  or  in  part  to  silicic 
acid  (H2SiO3) ;    all  of  which  substances  separate  as  white  amorphous 
precipitates  when  they  are  present  in  considerable  quantity.     Some 
nitrates,  especially  that  of  lead,  may  separate  in  crystalline  form  as 
the  acid  becomes  concentrated. 

3.  The  HG1  added  serves,  both  because  of  the  formation  of  aqua 
regia  and  because  of  its  own  specific  action,  to  bring  into  solution 
certain  alloys,  especially  those  of  iron  and  of  aluminum,  which  are 
only  slowly  attacked  by  HN03.    The  HC1  also  dissolves  any  oxide  of 
antimony  or  of  tin  which  may  have  been  produced  by  the  HNOj.    It 
may  cause  the  precipitation  of  lead  and  silver  as  chlorides. 

4.  The  heating  of  the  dry  residue  at  100-130°  serves  to  dehydrate 
silicic  acid  and  make  it  insoluble  in  acid.    This  heating  may  ordinarily 
be  omitted  except  in  the  case  of  iron  alloys,  since  these  are  the  only 
alloys  likely  to  contain  silica. 

5.  A  residue  undissolved  by  the  dilute  HC1  may  consist  of  silica, 
silicon,  carbon,  or  silver  chloride,  or  of  certain  alloys,  like  ferrochrome 
or  ferrosilicon,  which  are  only  slowly  attacked  by  HNOa  and  HC1. 
The  residue  is  treated  with  HF  and  H2SO4  by  P.  5  to  detect  and  re- 
move silica  and  to  bring  the  other  substances  into  solution. 

6.  The  statements  in  Notes  5-9,  P.  3,  are  applicable  also  to  alloys. 


48  PREPARATION  OF  THE  SOLUTION  P.  5 

Procedure  5.  —  Treatment  of  the  Residue  with  Hydrofluoric 
Acid.  —  Transfer  the  residue  undissolved  by  acids  (P.  3  or  4) 
to  a  platinum  or  palladium-gold  crucible  (see  Notes  i  and  2). 
Add  to  it  just  3  cc.  of  18  n.  H2SO4,  heat  the  crucible  with  a 
moving  flame  till  thick  white  fumes  begin  to  be  given  off,  and 
let  it  cool  completely. 

To  test  for  silicate,  add  carefully  from  a  lead  tube  or  hard- 
rubber  tube  capped  with  a  rubber  nipple  pure  48%  HF  drop 
by  drop  until  5-6  drops  have  been  added,  and  warm  the  mixture 
over  a  steam-bath.  (Formation  of  gas  bubbles,  presence  of 

SILICA  Or  SILICATE.) 

Then  add  2-5  cc.  more  pure  48%  HF,  place  the  cover  on  the 
crucible,  and  digest  the  mixture  on  a  steam-bath  for  about  15 
minutes  unless  the  residue  dissolves  more  quickly ;  remove  the 
cover;  evaporate  the  mixture  under  a  hood  till  white  fumes 
of  H2S04  begin  to  be  given  off,  taking  care  to  avoid  spattering 
(see  Note  3) ;  and  let  the  crucible  cool.  In  case  there  is  a 
residue  or  precipitate,  treat  the  mixture  as  described  in  the 
next  paragraph.  In  case  there  is  no  residue  or  precipitate, 
evaporate  the  mixture  under  a  hood  just  to  dryness,  taking 
care  to  avoid  spattering  and  overheating  (see  Note  3).  If 
there  is  now  no  residue  (or  only  an  insignificant  one),  proceed 
no  further.  If  there  is  a  considerable  residue,  cool  the  crucible, 
add  to  it  just  3  cc.  of  18  n.  I^SCX,  heat  it  slowly  till  the  acid 
begins  to  fume  (not  allowing  much  of  it  to  evaporate),  cool 
the  crucible,  and  treat  the  mixture  as  described  in  the  next 
paragraph. 

Pour  the  contents  of  the  crucible  into  15  cc.  of  water,  rinsing 
out  the  crucible  with  the  resulting  solution.  Boil  the  mixture 
gently  for  4  to  5  minutes,  or  so  long  as  the  residue  seems  to  be 
dissolving ;  filter,  and  wash  the  residue  with  i  n.  H2S04,  reject- 
ing the  washings.  Treat  the  solution  by  P.  11-89,  with  such 
modifications  as  are  justified  by  the  knowledge  that  lead,  barium, 
and  strontium  are  not  present  in  it.  Treat  the  residue  by  P.  6 
in  case  it  came  from  a  non-metallic  substance ;  or  by  P.  7  in 
case  it  came  from  an  alloy  (see  note  10). 


P.  5  PREPARATION  OF   THE  SOLUTION  49 

Notes.  —  i.  A  student  using  this  procedure  for  the  first  time  should 
work  under  the  direct  supervision  of  an  instructor.  Great  care  must  be 
taken  not  to  breathe  the  fumes  of  HF  nor  to  get  it  on  the  hands;  for  it  is 
extremely  irritating  and  produces  dangerous  burns. 

2.  Whenever  a  residue  or  precipitate  has  to  be  transferred  from  a  filter 
to  a  crucible  in  which  it  is  to  be  ignited  or  fused,  it  is  best  to  roll  up  the 
part  of  the  filter-paper  to  which  most  of  the  residue  adheres,  wind  a  platinum 
wire  around  it  in  the  form  of  a  spiral,  dry  it  by  holding  it  above  a  small 
gas-flame,  and  then  heat  it  with  a  slanting  flame  till  the  carbon  is  all  burnt 
off,  holding  the  filter  and  residue  constantly  over  the  crucible  placed  on  a 
watch-glass. 

3.  When  a  liquid  is  to  be  evaporated  in  a  crucible,  it  is  well  to  heat  it 
within  a  larger  iron  crucible,  which  serves  as  an  air-bath.     The  smaller 
crucible  may  be  supported  upon  a  nichrome  triangle  set  into  holes  bored  in 
the  side  of  the  iron  crucible,  or  upon  a  circular  disk  of  asbestos-board  with 
a  round  hole  cut  out  in  the  middle  and  slots  cut  out  along  the  sides. 

4.  The  test  for  silica  or  silicate  depends  on  the  formation  of  SiF* 
gas,  which  is  insoluble  in  strong  H2SO4,  but  dissolves  in  water  in  the 
presence  of  HF  with  formation  of  fluosilicic  acid,  H2SiFe.     With  free 
silica  the  evolution  of  gas  takes  place  in  the  cold ;    but  with  slowly 
decomposing  silicates,  such  as  feldspar,  the  test  is  obtained  only  upon 
warming.     A  few  silicates  are  not  acted  upon  by  HF  and  H2SO4,  and, 
of  course,  do  not  show  the  test  for  silica  at  this  point.    The  test  is 
delicate  enough  to  enable  i  mg.  of  silica,  whether  free  or  in  a  decompos- 
able silicate,  to  be  detected.     Moreover,  after  the  substance  has  been 
treated  with  acids  as  in  P.  3  or  4  and  heated  with  H2SO4,  an  evolution  of 
gas  with  HF  is  not  produced  with  the  compounds  of  any  element  other 
than  silicon.     It  should  be  borne  in  mind  that  a  small  quantity  of 
silica  will  be  introduced  if  ordinary  filters  (which  have  not  been  washed 
with  HF)  have  been  employed  and  have  been  destroyed  by  acids  or  by 
ignition,  or  if  a  strongly  alkaline  solution  has  been  boiled  in  glass  vessels. 

5.  Since  glass  and  porcelain  consist  of  silicates  which  are  readily 
attacked  by  HF,  this  acid  must  not  be  allowed  to  come  into  contact 
with  these  materials.    In  handling  cold  HF  solutions,  vessels  and  fun- 
nels of  celluloid  or  paraffin  or  of  glass  coated  with  paraffin  may  be  used ; 
but  platinum  or  palladium-gold  vessels  must  be  employed  when  the 
solutions  are  to  be  heated.     Care  must  be  taken  not  to  introduce  into 
a  platinum  or  palladium-gold  vessel  any  solution  containing  chlorine 
or  bromine  or  any  acid  mixture  containing  nitrates  and  chlorides  by 
which  chlorine  would  be  evolved.     Platinum  is  so  slowly  attacked  by 
hot  concentrated  H2SO4  that  even  when  2-3  cc.  of  the  acid  are  rapidly 
evaporated  in  a  crucible  less  than  0.5  mg.  passes  into  solution. 


50  PREPARATION  OF  THE  SOLUTION  P.  5 

6.  The  digestion  with  HF  decomposes  most  silicates  and  dissolves 
silica.    The  subsequent  evaporation  with  H2SO4  expels  the  excess  of 
HF  and  decomposes  the  fluorides  produced,  as  well  as  some  other 
substances  that  may  have  been  left  undissolved  by  the  HNO3  and  HC1. 

7.  In  case  there  is  no  residue  or  precipitate  after  evaporating  off 
the  HF  and  cooling  the  remaining  H2S04,  the  residue  undissolved  by 
dilute  HC1  (in  P.  3  or  4)  may  have  consisted  only  of  silica,  sulfur,  or 
carbon ;  in  which  case  it  is  unnecessary  to  analyze  the  H2SO4  solution 
further.    To  determine  this,  the  H2SO4  is  completely  evaporated  off; 
and,  if  there  is  still  no  solid  residue,  it  proves  that  the  solution  con- 
tains no  basic  constituents. 

8.  Just  3  cc.  of  1 8  n.  H2SO4  are  added  and  the  acid  is  not  allowed 
to  evaporate  in  the  subsequent  heating,  so  as  to  enable  the  acid  con- 
centration to  be  properly  adjusted  in  the  subsequent  H2S  precipitation. 
This  quantity  of  H2S04  (54  milliequivalents)  is  made  somewhat  larger 
than  the  quantity  (30  milliequivalents)  of  HNO3  or  HC1  added  hi 
P.  2,  3,  or  4,  in  order  to  allow  for  some  loss  in  the  evaporation  and  for 
the  smaller  degree  of  ionization  of  HS04~.     Only  15  cc.  of  water  are 
added,  so  as  to  prevent  the  precipitation  of  antimony  and  bismuth  as 
oxysalts,  and  so  as  to  cause  the  complete  precipitation  of  lead,  barium, 
and  strontium  as  sulfates.    The  solution  is  boiled  so  as  to  dissolve 
anhydrous  sulfates,   especially   those  of  iron   and   aluminum.    The 
residue  is  washed  with  i  n.  H2SO4  so  that  PbS04  and  SrSO4  may  not 
be  dissolved. 

9.  The  residue  undissolved  by  dilute  H2S04  contains  as  sulfates  all 
of  any  barium,  strontium,  or  lead,  and  nearly  all  of  any  calcium  or 
chromium  that  remained  hi  the  residue  from  the  treatments  with  HNO3 
and  HC1.    The  chromium  may  be  present  because  it  is  converted  into 
an  anhydrous,  slowly  dissolving  sulfate.    The  residue  may  also  con- 
tain some  bismuth  as  basic  sulfate,  and  some  antimony  as  antimonic 
oxide.     In  it  may  also  be  present  still  undecomposed  substances, 
especially  the  following :  silver  chloride ;  corundum  (A12O3) ;  chromite, 
(FeCr204) ;  cassiterite  (SnO2) ;  some  anhydrous  silicates,  such  as  cyanite 
or  andalusite  (Al2SiO6)   and  tourmalin;    graphite  and  carborundum 
(SiC). 

10.  In  the  case  of  an  alloy  any  residue  undissolved  by  the  dilute 
H2SO4  probably  consists  only  of  graphite,  or  of  some  of  the  silver 
chloride,  chromium  sulfate,  or  original  alloy  which  has  escaped  de- 
composition.   If  black,  it  may  be  tested  for  graphite  by  drying  it  and 
rubbing  it  on  the  fingers  or  on  paper.     Unless  it  seems  to  consist 
wholly  of  graphite,  it  is  treated  by  P.  7,  and  the  solutions  thus  obtained 
are  tested  only  for  silver  and  chromium. 


P.  6  PREPARATION  OF  THE  SOLUTION  51 

it.  If  the  vise  of  a  platinum  or  a  palladium-gold  crucible  or  of 
hydrofluoric  acid  is  impracticable,  the  residue  insoluble  in  HC1  may  be 
fused  in  a  nickel  crucible  with  Na2C03,  as  described  in  P.  7,  instead  of 
being  treated  by  P.  5-6.  This  is,  however,  a  far  less  satisfactory 
method  of  analysis  for  the  following  reasons.  Compounds  of  the 
alkali  elements  are  used  as  a  flux,  nickel  is  introduced  from  the  crucible, 
and  mercury  compounds  are  volatilized ;  so  these  elements  cannot  be 
tested  for  in  the  subsequent  analysis.  Moreover,  the  treatment  with 
HF  and  H2SO4  is  almost  always  a  shorter  process,  since  when  the  residue 
consists  only  of  silica,  as  is  often  the  case  with  minerals,  no  further 
treatment  is  necessary,  and  since  in  other  cases  there  is  often  no  residue 
to  be  boiled  with  Na2CO3  solution  (P.  6). 

Procedure  6.  —  Treatment  of  the  Residue  from  the  Fluoride 
Treatment.  —  Transfer  the  residue  undissolved  by  dilute  H2SO4 
(P.  5)  to  a  casserole,  add  about  25  cc.  of  3  n.  Na2CO3  solution, 
cover  the  casserole,  and  boil  gently  for  10  minutes.  Filter  and 
wash  the  residue  thoroughly.  (Filtrate,  reject.)  Heat  the 
residue  with  just  5  cc.  of  HC1  and  10  cc.  of  water,  and  filter  the 
boiling-hot  solution  if  there  is  still  a  residue.  Treat  the  solution 
by  P.  21-22,  31-35,  51-57,  and  71-77  to  test  for  lead,  bismuth, 
chromium,  barium,  strontium,  and  calcium. 

In  case  the  HC1  left  a  residue,  treat  a  fresh  i  g.  sample  of  the 
substance  by  P.  2  and  3,  and  treat  the  residue  so  obtained  by  P.  7. 

Notes.  —  i.  The  boiling  with  Na2CO3  converts  into  carbonates  the 
sulfates  of  lead,  calcium,  strontium,  and  bismuth  completely,  and  at 
least  80  per  cent  of  the  sulfate  of  barium,  even  when  large  quantities 
of  them  are  present.  A  second  treatment,  which  should  be  applied 
to  the  residue  if  there  are  indications  that  barium  is  present,  completely 
decomposes  BaSO4.  The  carbonates  dissolve  readily  in  hot  HC1. 
Anhydrous  chromic  sulfate,  which  is  left  undissolved  by  dilute  H2S04 
(P.  5)  as  a  fine  pink  or  gray  powder,  is  slowly  changed  by  boiling  with 
Na2COs  to  a  greenish  blue  hydroxide  which  dissolves  in  the  HC1,  leav- 
ing behind  the  still  undecomposed  sulfate.  Antimonic  oxide  dissolves 
only  to  a  small  extent  (2-4  mg.)  in  the  Na2CO3  solution,  but  dis- 
solves in  the  dilute  HC1.  Silver  chloride  is  only  slightly  attacked  by 
NasCO2  solution. 

2.  Any  residue  insoluble  in  HC1  can  therefore  consist  only  of  barium 
or  chromic  sulfate,  of  silver  chloride,  or  of  some  of  the  original  sub- 
stance still  undecomposed,  which  is  likely  to  consist  of  one  of  the  native 


52  PREPARATION  OF  THE  SOLUTION  P.  7 

oxides  or  silicates  mentioned  in  Note  9,  P.  5.  If  such  a  residue  is 
obtained,  it  can  ordinarily  be  rendered  soluble  by  fusion  with  NaaCOa 
and  NaN03,  as  described  in  P.  7.  It  is,  however,  preferable  to  treat 
a  fresh  sample  of  the  substance  with  HNO3  and  HC1  (by  P.  2  and  3), 
and  to  fuse  the  residue  from  that  treatment  with  Na2CO3  and  NaN03 ; 
for  this  makes  it  possible  to  detect  in  the  aqueous  extract  of  the  fused 
mass  certain  acidic  constituents,  namely,  fluoride,  borate,  and  sulfate, 
which  might  otherwise  escape  detection. 

Procedure  7.  —  Fusion  of  the  Residue  with  Sodium  Carbonate. 
—  In  case  fusion  with  Na2C03  is  to  be  substituted  for  the  HF 
treatment  (P.  5),  treat  the  residue  undissolved  by  HNO3  and 
HC1  (P.  3  or  4)  as  described  in  the  following  paragraph.  Or, 
in  case  HF  has  been  used  and  HC1  has  left  a  residue  in  P.  6, 
and  if  a  fresh  sample  of  the  substance  has  been  treated  by  P.  2 
and  3  (as  directed  in  P.  6),  treat  the  residue  so  obtained  as  de- 
scribed in  the  following  paragraph. 

Transfer  the  residue  (see  Note  2,  P.  5)  to  a  30  cc.  nickel 
crucible,  mix  it  thoroughly  with  10-20  cc.  of  anhydrous  Na2C03, 
cover  the  crucible,  heat  it  strongly  over  a  powerful  burner, 
preferably  within  a  cylinder  of  asbestos  paper,  so  that  complete 
fusion  takes  place,  and  continue  the  heating  for  10-20  minutes. 
If  dark  particles  of  undecomposed.  substance  can  still  be  seen, 
add  gradually  in  small  portions  0.1-0.3  cc-  of  s°lid  NaNO3,  and 
heat  strongly  for  several  minutes.  Cool  the  crucible,  place  it 
in  a  casserole  with  40-60  cc.  of  water,  and  boil  the  mixture  till 
the  fused  mass  is  disintegrated.  Filter,  and  wash  the  residue 
thoroughly. 

Treat  one  half  of  the  aqueous  extract  as  directed  in  P.  123,  to 
detect  acidic  constituents ;  and  treat  the  other  half  as  described 
in  the  next  paragraph. 

Rinse  the  residue  not  dissolved  by  water  into  a  casserole 
with  15  cc.  of  2  n.  HNO3,  boil  the  mixture  gently  so  long  as  the 
residue  seems  to  be  dissolving,  and  filter  out  and  reject  any 
undecomposed  substance.  Mix  one  tenth  of  this  HNOs  solu- 
tion with  one  fifth  of  the  half-portion  of  the  aqueous  extract, 
and  acidify  the  mixture  with  HN03.  If  no  precipitate  forms, 
mix  the  rest  of  the  HN03  solution  with  the  remaining  four- 


P.  7  PREPARATION  OF  THE  SOLUTION  53 

fifths  of  the  half-portion  of  the  aqueous  extract,  acidify  the  mix- 
ture, and  treat  it  as  described  in  the  following  paragraph.  If 
a  precipitate  forms  on  mixing  the  HNOs  solution  and  the 
aqueous  extract,  reject  the  mixed  portions,  and  treat  the  re- 
maining portions  of  the  two  solutions  separately  as  described  in 
the  following  paragraph,  uniting  the  precipitates  formed  by  the 
same  group-reagent  in  the  subsequent  analysis. 

Add  5  cc.  of  12  n.  HC1,  and  filter.  Treat  the  precipitate  by 
P.  12-13,  to  test  for  lead  and  silver.  Evaporate  the  filtrate  to 
dryness,  and  heat  the  residue  till  it  becomes  perfectly  dry,  by 
keeping  the  casserole  in  motion  over  a  small  flame.  Add  just 
5  cc.  of  6  n.  HC1  and  10  cc.  of  water,  heat  the  mixture  to  boil- 
ing, and  filter  it.  Wash  the  residue,  and  treat  it  by  the  first  two 
paragraphs  of  P.  5,  to  confirm  the  presence  of  silica.  Treat  the 
filtrate  by  P.  21-79,  to  detect  basic  constituents. 

Notes.  —  i.  Upon  fusion  with  sodium  carbonate  most  compounds 
undergo  metathesis,  the  acidic  constituent  of  the  compound  combining 
with  the  sodium  and  the  basic  element  with  the  carbonate.  The 
carbonate  formed  is,  however,  sometimes  decomposed  by  heat  with 
production  of  the  oxide  or  of  the  metal  itself.  Acid-forming  oxides, 
such  as  Si02,  As2Os,  and  less  rapidly  A12O3,  expel  CO2  from  the  carbonate 
and  form  sodium  salts.  Such  reactions  are  illustrated  by  the  following 
equations : 

BaS04+Na2C03  =Na2SO4  -|-BaC03. 

Fe2Si06+Na2C03=Na2Si03+Fe203+C02. 

4  AgCl+2  Na2C03=4  NaCl  +4  Ag+2  CO2+02. 
Si02+Na2CO3 = Na2SiOs+  C02. 

2.  The  NaNOs  serves  to  oxidize  some  substances  which  are  not 
much  acted  upon  by  Na2CO3  alone.     Thus  sulfides  are  converted  into 
sulfates;   chromium  compounds,  such  as  Cr2(SO4)3  or  FeOCr203  (the 
mineral  chromite),  into  chromates;    and  manganese  compounds  into 
manganates. 

3.  After  the  treatment  with  water  all  the  acidic  constituents  of  the 
substance  are  found  in  the  aqueous  extract  as  sodium  salts,  and  a 
portion  of  this  extract  is  therefore  reserved  to  be  tested  for  these  con- 
stituents.    Certain  of  the  basic  constituents,  namely  arsenic,  antimony, 
tin,  aluminum,  chromium,  and  manganese,  may  pass  wholly  or  in  part 
into  the  aqueous  extract;    and  a  part  of  this  solution  is  therefore 
analyzed  for  basic  constituents.    Most  of   the  basic   constituents, 


54  PREPARATION  OF  THE  SOLUTION  P.  8 

however,  remain  in  the  residue  from  the  aqueous  extract,  and  are 
dissolved  by  the  dilute  HNO3. 

4.  To  save  time,  the  HNO3  solution  and  the  portion  of  the  aqueous 
extract  to  be  tested  for  basic  constituents  are  mixed  whenever  this 
can  be  done  without  producing  a  precipitate.    Whether  a  precipitate 
results  is  determined  by  the  preliminary  test  with  small  portions  of  the 
solutions.    The  first  and  third  equations  hi  Note  i  are  examples  of 
cases  in  which  the  mixing  would  result  in  a  precipitate. 

5.  HC1  is  added  before  the  evaporation  to  precipitate  silver  and 
lead.    A  considerable  quantity  is  added  to  destroy  the  nitrates,  since 
the  residue  obtained  by  the  subsequent  evaporation  is  more  soluble 
when  in  the  state  of  chlorides. 

6.  The  evaporation  to  dryness  and  subsequent  heating  at  100-130° 
serves  to  render  silicic  acid  insoluble  (see  Note  6,  P.  3). 

7.  A  few  substances,  such  as  the  native  or  ignited  oxides  of  aluminum 
and  tin,  may  be  only  partially  attacked  even  by  long-continued  fusion 
with  NazCOa  and  NaNOs.     Some  silicates  also  may  not  be  completely 
decomposed,  especially  if  the  substance  was  not  very  finely  powdered ; 
but  enough  of  all  of  these  substances  is  brought  into  solution  to  secure 
their  detection.     A  residue  from  the  HNOs  treatment  may  therefore 
ordinarily  be  rejected. 

8.  A  few  milligrams  of  nickel  are  taken  up  from  the  crucible  by  the 
flux,  so  that  this  element,  as  well  as  the  alkali  elements,  cannot  be 
tested  for  later  in  the  analysis.    The  crucible  is,  however,  so  little 
attacked  by  the  flux  that  it  can  be  used  repeatedly. 

9.  Whenever  it  is  permissible,  it  is  somewhat  better  to  make  the 
fusion  in  a  platinum  crucible,  since  then  no  foreign  substances  are 
introduced  from  the  crucible.    It  is  not  permissible,  however,  to  ignite 
in  platinum  vessels  compounds  of  the  silver-,  copper-,  and  tin-groups ; 
for  these  may  be  reduced  to  the  metal  by  heating  with  an  alkaline  flux. 
The  same  is  true  of  sulfur,  sulfides,  and  in  the  presence  of  organic  matter 
of  phosphates;   for  all  these  elements  form  easily  fusible  alloys  with 
the  platinum,  and  thus  spoil  the  crucible.     Moreover,  alkaline  hy- 
droxides and  strongly  oxidizing  fluxes  (such  as  peroxides  and  nitrates) 
must  not  be  fused  in  platinum,  since  they  attack  it  fairly  rapidly. 
Therefore,  if  the  fusion  is  made  in  platinum,  no  more  NaN03  should 
be  added  than  is  necessary. 

Procedure  8.  —  Treatment  of  Substances  Containing  Organic 
Matter.  —  If  the  closed-tube  test  (P.  i)  has  shown  the  presence 
of  organic  matter,  powder  or  cut  into  small  pieces  1-5  g.  of  the 
substance  (according  to  the  quantity  of  organic  matter  present). 


P.  8  PREPARATION  OF  THE  SOLUTION  55 

Add  to  it  in  a  casserole  about  10  cc.  of  18  n.  H2SO4;  heat  grad- 
ually until  the  substance  is  well  charred;  cool;  add  slowly, 
with  constant  stirring,  under  a  hood,  16  n.  HNO3,  until  violent 
reaction  ceases ;  warm  gently  for  a  few  minutes,  and  then  heat 
more  strongly,  keeping  the  dish  moving,  until  the  substance  is 
thoroughly  charred.  Cool,  again  add  16  n.  HN03  as  before, 
and  heat  until  thick  fumes  of  H2S04  are  evolved.  Repeat  this 
process  till  the  mixture  becomes  light-colored  and  remains  so 
when  heated  strongly. 

In  case  the  substance  has  dissolved  completely,  evaporate 
the  mixture  carefully  under  a  hood  just  to  dryness,  let  the  dish 
cool,  and  pour  into  it  just  8  cc.  of  6  n.  H2SO4  and  10  cc.  of  water. 
If  there  is  a  residue,  boil  the  mixture  so  long  as  it  seems  to  be 
dissolving,  filter  out  any  residue  still  remaining,  wash  it,  and 
treat  it  by  P.  6.  Treat  the  solution  by  P.  n. 

In  case  the  substance  has  not  dissolved  completely,  transfer 
the  mixture  to  a  platinum  or  palladium-gold  crucible,  let  it 
cool,  and  treat  it  (with  HF)  as  described  in  the  last  three  para- 
graphs of  P.  5.  (If  the  use  of  a  platinum  crucible  or  of  HF  is 
not  practicable,  treat  the  mixture  as  described  in  the  foregoing 
paragraph,  except  that  the  residue  undissolved  by  the  dilute 
H2SO4  should  be  treated  by  P.  7,  instead  of  by  P.  6.) 

Notes.  —  i.  This  method  of  destroying  organic  matter  is  of  very 
general  application,  being  effective  even  when  such  stable  substances 
as  paraffin  and  cellulose  are  present.  Organic  matter  can  also  be 
destroyed  by  ignition ;  but  this  has  the  disadvantages  of  volatilizing 
certain  elements,  especially  mercury  and  arsenic,  and  of  making  some 
substances  very  difficultly  soluble.  When  the  organic  matter  consists 
only  of  oil,  as  is  the  case  with  an  oil  paint,  it  may  advantageously  be 
extracted  with  ether. 

2.  In  case  the  substance  dissolved  completely  in  the  concentrated 
H2SO4,  it  shows  the  absence  of  silica  and  silicates ;  for  the  silicic  acid 
liberated  by  the  decomposition  of  any  silicate  is  dehydrated  and  made 
insoluble  by  heating  with  strong  H2S04.  In  this  case  the  treatment 
with  HF  is  omitted;  and  the  solution  is  evaporated  to  remove  the 
large  quantity  of  H2SO4  so  that  it  may  not  interfere  with  the  H2S 
precipitation,  and  the  residue  is  treated  with  the  proper  quantity  of 
dilute  HtSO4.  A  small  residue  of  sulfate  of  lead,  barium,  strontium, 


56  PREPARATION  OF  THE  SOLUTION  P.  9 

or  calcium,  or  of  oxide  of  antimony,  may  then  remain ;  and  any  such 
residue  is  treated  by  P.  6,  to  bring  these  elements  in  solution.  If  there 
is  no  such  residue,  it  shows  the  absence  of  lead,  barium,  and  strontium 
in  the  substance. 

3.  In  case  the  substance  did  not  dissolve  completely  in  concen- 
trated H2SO4,  the  mixture  is  treated  with  HF  to  detect  and  remove 
silica.  There  may  still  be  a  residue  undissolved  by  the  dilute  HjSO*. 
This  will  contain  any  substances  originally  present  that  have  not  been 
attacked  by  HN03,  H2S04,  or  HF ;  all  the  lead,  strontium,  and  barium 
that  may  have  been  present  in  any  form,  since  the  sulfates  of  these 
elements  are  very  slightly  soluble  in  dilute  H2SO4 ;  some  of  the  calcium, 
bismuth,  antimony,  and  tin,  when  these  elements  are  present  in  con- 
siderable quantity,  since  their  sulfates  (or  oxides)  are  not  readily  dis- 
solved by  dilute  H2S04 ;  and  substantially  all  of  the  chromium,  since 
its  sulfate  is  converted  into  the  insoluble  anhydrous  form  by  heating 
with  strong  H2S04. 

Procedure  9.  —  Treatment  of  Solutions.  —  If  the  substance  is 
a  solution  in  water  or  other  volatile  solvent,  test  its  effect  upon 
litmus  paper;  and  evaporate  10  cc.  of  it  (after  adding  to  it 
NH4OH  to  alkaline  reaction  if  it  reddened  litmus  paper)  in  a 
small  weighed  dish  to  complete  dryness  over  a  steam-bath,  so 
as  not  to  overheat  the  residue ;  and  weigh  the  dish  again.  Treat 
the  residue  or  a  portion  of  it  by  the  first  paragraph  of  P.  i  (heat- 
ing it  in  a  closed  tube)  to  detect  organic  matter. 

In  case  organic  matter  is  proved  absent,  neutralize  such  a 
volume  of  the  solution  as  contains  i  g.  of  solid  substance  exactly 
with  HNOs  or  NH.iOH,  bring  the  mixture  to  a  volume  of  10  cc. 
by  evaporation  or  dilution,  and  add  to  it  5  cc.  of  6  n.  HNOs. 
If  no  precipitate  has  separated  from  the  solution,  treat  it  by 
P.  11-89,  to  detect  basic  constituents.  If  a  precipitate  has 
separated,  treat  the  mixture  by  P.  3  (and,  if  necessary,  by 
P.  5-6  or  7),  followed  by  P.  21-89,  to  detect  basic  constituents. 

In  case  organic  matter  is  found  present,  evaporate  to  dryness 
such  a  volume  of  the  solution  as  contains  i  g.  of  solid  substance, 
and  treat  the  residue  as  directed  in  P.  8,  to  detect  basic  con- 
stituents. 

In  either  case  add  to  such  a  volume  of  the  solution  as  contains 
2.5  g.  of  solid  substance  25  cc.  of  3  n.  Na2C03  solution,  evaporate 


P.  9  PREPARATION  OF  THE  SOLUTION  57 

the  mixture  to  just  30  cc.,  filter  it,  and  treat  portions  of  the 
nitrate  by  P.  102-116,  to  detect  acidic  constituents. 

Treat  another  portion  of  the  solution,  to  detect  carbonate 
and  ammonium,  as  follows :  Place  5-30  cc.  of  the  solution  in  a 
distillation  apparatus  arranged  as  in  P.  117,  pour  in  through 
the  safety-tube  5  cc.  of  HC1,  distil  off  2-3  cc.  of  liquid,  and  test 
the  distillate  for  carbonate  as  in  P.  117.  Make  the  mixture  in 
the  flask  alkaline  with .  NaOH,  distil  off  5  cc.  of  the  liquid  into 
5  cc.  of  water,  and  test  the  distillate  for  ammonium  as  in  the 
second  paragraph  of  P.  91. 

Notes.  —  i.  A  10  cc.  portion  of  the  solution  is  evaporated  to  dry- 
ness  and  the  residue  is  weighed,  to  determine  how  much  dissolved  sub- 
stance is  present  and  thus  enable  the  usual  amount  of  substance  to  be 
taken  for  analysis.  NH<OH  is  added  when  the  solution  is  acid,  to  pre- 
vent the  loss  of  volatile  acids. 

2.  The  solution,  rather  than  the  solid  residue  obtained  from  it,  is 
tested  for  carbonate  and  for  ammonium,  since  these  constituents  might 
be  lost  on  evaporation. 


DETECTION   OF  THE  BASIC   CONSTITUENTS 

GENERAL  DISCUSSION 

THE  science  of  qualitative  chemical  analysis  treats  of  the 
methods  of  determining  the  nature  of  the  elements  and  of  the 
chemical  compounds  which  are  present  in  any  given  substance. 
When  the  presence  or  absence  of  the  various  elements  is  alone 
determined,  the  process  is  called  ultimate  analysis ;  when  the 
chemical  compounds  of  which  the  substance  is  composed  are 
identified,  it  is  called  proximate  analysis.  In  the  analysis  of 
inorganic  substances,  to  which  this  book  is  devoted,  the  object 
in  view  is  ordinarily  an  intermediate  one  —  namely,  that  of 
detecting  the  base-forming  and  acid-forming  constituents  (called 
in  this  book  simply  the  basic  and  acidic  constituents)  that  are 
present  in  the  substance.  Thus  the  analysis  of  a  substance 
consisting  of  calcium  sulfate,  zinc  chromate,  and  ferric  oxide 
would  show  not  only  that  the  elements  calcium,  sulfur,  zinc, 
chromium,  and  iron  were  present,  but  also  that  the  sulfur  was 
in  the  form  of  sulfate  (not  sulfide  or  sulfite),  the  chromium  in 
the  form  of  chromate  (not  of  a  chromic  salt),  and  the  iron  in 
the  ferric  (not  the  ferrous)  state.  The  reason  for  this  is  that  the 
analysis  is  carried  out  by  dissolving  the  substance  in  water 
(with  aid  of  acids,  if  necessary),  and  by  treating  the  solution 
so  obtained  successively  with  a  number  of  different  chemical 
substances.  Now,  since  the  chemical  reactions  of  substances 
in  aqueous  solutions  are  determined  by  the  nature  of  the  ions 
which  they  yield,  and  since  the  ions  correspond  to  the  basic 
and  acidic  constituents,  it  is  these  constituents  whose  presence 
or  absence  is  established. 

For  detecting  the  basic  constituents  a  systematic  method  is 
employed  which  consists  in  adding  to  an  acid  solution  of  the 
substance  in  succession  ammonium  chloride,  hydrogen  sulfide, 
ammonium  hydroxide  and  sulfide,  and  ammonium  carbonate. 

58 


DETECTION  OF  THE  BASIC  CONSTITUENTS  59 

By  each  of  these  reagents  a  group  of  basic  constituents  is  pre- 
cipitated. Thus,  ammonium  chloride  precipitates  those  constit- 
uents whose  chlorides  are  only  slightly  soluble  in  water ;  hydro- 
gen sulfide,  those  whose  sulfides  are  only  slightly  soluble  in  dilute 
acid ;  ammonium  hydroxide  and  sulfide,  those  whose  sulfides  or 
hydroxides  are  only  slightly  soluble  in  ammoniacal  solutions; 
and  ammonium  carbonate,  those  whose  carbonates  are  only 
slightly  soluble  in  water  containing  ammonium  carbonate.  The 
way  in  which  the  basic  constituents  are  thus  separated  into 
groups  is  shown  in  more  detail  in  Table  II  on  the  following  page. 


60 


SEPARATION  INTO  GROUPS 


Fil 


1 


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ft  S 
§3 


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^2 


till 

«    HH      W    < 
fi 


»o  3 

S    !5      § 

2fi 

II 

AH 


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O    w         c/3 
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P.  11  PRECIPITATION  OF  SILVER-GROUP  6l 

PRECIPITATION   AND   ANALYSIS   OF   THE   SILVER-GROUP 


TABLE  III.  —  ANALYSIS  OF  THE  SILVER-GROUP. 


Precipitate :   PbCl2,  AgCl,  Hg2Cl2.     Treat  with  hot  water  (P.  12). 


Solution:   PbCl2. 
Add  K2CrO<  (P.  12]. 

Residue  :  AgCl,  Hg2Cl2. 
Pour  NHtOH  through  the  filter  (P.  13). 

Precipitate:  PbCrO4 

Black  residue  : 
Hg  and  Hg^ 

Solution  : 
Ag(NH3)2Cl. 
Add  HN03  (P.  /j). 

Precipitate  :  AgCl. 

Procedure  n.  —  Precipitation  of  the  Silver-Group.  —  Pour  the 
cold  solution  of  the  substance  (prepared  by  P.  2  or  by  P.  5-9, 
and  containing  30  milliequivalents  of  HNOs  or  about  50  milli- 
equivalents  of  H^SCX  in  about  15  cc.  of  solution)  into  a  conical 
flask  (see  Note  i),  and  add  to  it  4  cc.  of  3  n.  Nt^Cl  solution 
(see  Notes  2  and  3).  (White  precipitate,  presence  of  SILVER- 
GROUP.)  Let  the  mixture  stand  for  3  or  4  minutes ;  then  filter 
it.  Wash  the  precipitate  with  5-10  cc.  of  cold  2  n.  HC1,  re- 
jecting the  washings.  (Precipitate,  P.  12;  nitrate,  P.  21.) 

Notes.  —  i.  //  is  recommended  that  in  general  hard-glass  conical 
flasks  (the  so-called  Erlenmeyer  flasks  of  hard  glass),  rather  than  beakers 
or  test-tubes,  be  employed  for  holding  solutions  that  are  being  subjected  to 
the  operations  of  precipitation  and  heating. 

2.  Even  in  cases  where  it  is  not  essential  to  add  a  perfectly  definite 
volume  of  a  reagent,  the  analyst  should  make  it  a  practice  to  measure  out 
the  quantity  to  be  added,  rather  than  to  pour  in  an  indefinite  quantity 
from  the  reagent  bottle.  For  this  purpose  a  10  cc.  graduate  should  be 
constantly  at  hand.  For  adding  smaller  quantities  than  2  cc.  a  dropper 
should  be  used.  This  may  be  made  by  drawing  out  one  end  of  a  short  glass 
tube  to  a  wide  capillary  and  capping  the  other  end  with  a  rubber  nipple. 
When  more  of  a  reagent  than  is  needed  has  been  poured  into  a  graduate 
or  other  vessel,  it  should  never  be  poured  back  into  the  reagent  bottle,  owing 
to  the  danger  of  contaminating  the  reagent. 


62  PRECIPITATION*  OF  SILVER-GROUP  P.  11 

3.  Unless  the  concentration  is  specified,  as  is  done  in  this  case,  it  is 
understood  that  all  salt  solutions  used  as  reagents  are  i  normal  (i  «.), 
that  is,  that  they  contain  one  equivalent  of  salt  per  liter  of  solution;  also 
that,  unless  otherwise  specified,  the  acid  and  base  solutions  used  as  reagents 
are  6  normal  (6  «.)• 

4.  By  one  equivalent  of  any  substance  is  meant  that  weight  of  it 
which  reacts  with  one  atomic  weight  (1.008  grams)  of  hydrogen  in 
any  of  its  compounds  or  with  the  weight  of  any  other  substance  which 
itself  reacts  with  one  atomic  weight  of  hydrogen.    Thus,  one  equiva- 
lent is  the  quantity  in  grams  corresponding  to  the  following  formulas : 
i  NaOH,  |  Ba(OH)2,   i  HC1,  \  H2SO4,  \  H3P04,  i  NH4C1,  \  Na2SO4, 
\  CaSO4,  \  FeCl3.    The  equivalent  weight  of  a  substance  is  evidently 
not  identical  with  its  formula-weight,  by  which  is  meant  the  number  of 
grams  represented  by  its  formula;    thus  one  equivalent  of  H2SO4  is 
49.04  grams,  but  one  formula- weight  is  98.08  grams.    When  a  substance 
may  take  part  either  in  a  reaction  of  metathesis  or  in  one  of  oxidation 
and  reduction,  its  metathetical  equivalent  has  to  be  distinguished  from 
its  oxidation  equivalent.    Thus,  the  metathetical  equivalent  of  nitric 
acid  is  i  HNO3 ;   but  its  oxidation  equivalent  (when  it  is  reduced  to 
NO)  is  f  HNO3.    In  this  book  the  term  equivalent  will  always  be  used 
to  denote  the  metathetical  equivalent.       Note  that  the  number  of 
equivalents  of  a  substance  is  a  certain  quantity  of  it ;   but  that  the 
terms  normal  and  formal  denote  its  concentration,  that  is,  the  quantity 
of  it  per  unit-volume;   normal  signifying  the  number  of  equivalents 
of  it  per  liter,  and  formal,  the  number  of  formula-weights  per  liter. 

5.  If  NH4C1  produces  no  precipitate,  it  proves  the  absence  of  silver 
and  mercurous  mercury,  but  not  of  lead,  since  PbCl2  is  fairly  soluble 
in  water  even  in  the  presence  of  chlorides.    Thus,  under  the  conditions 
of  this  Procedure  not  more  than  50  mg.  of  lead  remain  in  solution. 

6.  The  solubility  of  PbCl2  is  much  smaller  in  a  solution  of  NH4C1 
or  of  any  other  chloride  than  it  is  in  water,  owing  to  the  so-called 
common-ion  effect,  which  may  be  explained  in  detail  as  follows :  The 
mass-action  law  requires  that  at  a  given  temperature  in  all  dilute  solu- 
tions containing  lead  chloride  the  ratio  of  the  product  of  the  ion- 
concentrations*  (Pb++)X(Cl-)2  to  the  concentration  (PbCl2)  of  the 
unionized  salt  have  the  same  value ;  that  is,  (Pb++)  X  (C1~)2-T-  (PbCl2)  = 
some  definite  value.    Now  hi  all  solutions  which  have  been  saturated 
with  lead  chloride  as  a  result  of  sufficiently  long  contact  with  the  solid 
substance,  the  concentration  of  the  lead  chloride  present  as  such  (that 
is,  as  unionized  PbCl2)  must  evidently  have  the  same  value,  and  there- 

*  In  mass-action  expressions  of  this  kind,  chemical  formulas  within  parentheses 
denote  the  concentrations  of  the  respective  substances,  that  is,  the  quantities  of  them 
per  liter  of  solution. 


P.  ll  PRECIPITATION  OF  SILVER-GROUP  63 

fore  in  all  such  saturated  solutions  the  ion-concentration  product 
(Pb++)  X(C1~)2  must  also  have  the  same  value ;  that  is,  in  all  solutions 
saturated  at  a  given  temperature  with  lead  chloride,  (Pb++)X(Cl~)2= 
some  definite  value.  This  particular  value  which  the  ion-concentration 
product  has  when  the  solution  is  saturated  is  commonly  called  the 
solubility-product;  but  the  principles  involved  are  less  likely  to  be 
misunderstood  if  it  be  called  the  saturation-value  of  the  ion-concentration 
product.  The  saturation-value  varies,  of  course,  with  the  nature  of 
the  salt,  and  with  the  temperature  in  the  case  of  a  given  salt.  In  the 
case  of  lead  chloride  at  20°,  whose  solubility  in  water  at  20°  will  be 
seen  by  reference  to  the  Table  of  Solubilities  in  the  Appendix  to  be 
70  milliequivalents  per  liter,  the  saturation- value  of  the  ion-concen- 
tration product  in  millimols  per  liter  is  evidently  (35)X(7o)2=i7i,5oo, 
provided  the  ionization  be  considered  to  be  complete,  as  may  be  as- 
sumed to  be  true  in  these  qualitative  considerations  hi  the  case  of 
nearly  all  neutral  salts.  Any  solution  containing  lead-ion  and  chloride- 
ion  in  which  the  ion-concentration  product  exceeds  this  saturation- 
value  is  evidently  supersaturated  and  tends  to  deposit  the  solid  sub- 
stance ;  and  any  solution  in  which  the  ion-concentration  product  is 
less  than  the  saturation-value  is  evidently  undersaturated  and  tends 
to  dissolve  more  of  the  solid  substance.  Now,  when  NH4C1  or  HC1  is 
added  to  a  saturated  solution  of  PbCl2  in  water,  the  immediate  effect 
is  to  increase  the  value  of  (Cl~),  and  therefore  of  the  product  (Pb++)X 
(Q-)2;  but  the  solution  becomes  thereby  supersaturated,  and  PbCl2 
will  precipitate  out  of  it  until  the  saturation-value  of  the  product 
(Pb++)X(Cl-)2  is  restored. 

7.  Bismuth  and  antimony  might  be  precipitated  by  the  NH4C1 
as  oxychlorides  (BiOCl  and  SbOCl),  if  the  directions  for  preparing  the 
solution  of  the  original  substance  were  not  followed.    But  under  the 
prescribed  procedure,  which  yields  a  mixture  with  a  fairly  large  acid 
concentration,  these  elements  remain  in  solution  ;  for  their  oxychlorides, 
though  only  very  slightly  soluble  in  water,  dissolve  readily  in  sufficiently 
concentrated  acid. 

8.  The  precipitate  is  washed  with  2  n.  HC1,  rather  than  with  water, 
first,  in  order  that  bismuth  and  antimony  may  not  be  precipitated  as 
oxychlorides  in  the  filter,  and  secondly,  in  order  that  as  little  PbCl2 
as  possible  be  dissolved.    Only  a  small  volume  (5-10  cc.)  is  used,  so 
as  not  to  dissolve  much  PbCU.     Stronger  HC1  is  not  used,  since  it 
would  dissolve  more  PbCl2,  AgCl,  and  Hg2Cl2,  owing  to  the  formation 
of  acids  with  complex  anions,  such  as  H+2PbCU=,  H+iAgCk=,  and 
H+2HgCU=.    The  acid  washings  are  not  added  to  the  nitrate,  so  that 
the  acid  concentration  may  be  properly  adjusted  in  the  subsequent 
H2S  precipitation. 


64  ANALYSIS  OF  SILVER-GROUP  P.  13 

Procedure  12.  —  Extraction  and  Detection  of  Lead.  —  Pour 
repeatedly  through  the  filter  containing  the  NEUCl  precipitate 
(P.  n)  a  10  cc.  portion  of  boiling  water  (see  Note  i).  Wash 
the  residue  thoroughly  with  hot  water,  and  treat  it  by  P.  13. 
Cool  the  10  cc.  portion  of  water,  and  add  to  it  2  cc.  of  HN03 
and  2  cc.  of  3  n.  K2CrO4  solution  (see  Notes  2  and  3,  P.  u). 
(Yellow  precipitate,  presence  of  LEAD.) 

Notes.  —  i.  When  it  is  directed  to  dissolve  a  precipitate  by  pouring 
the  solvent  repeatedly  through  the  filter,  this  is  best  done  by  pouring  a  single 
portion  of  the  solvent  from  one  test-tube  through  the  filter  into  another 
test-tube,  back  and  forth,  three  or  four  times.  When  the  solvent  is  to  be 
used  hot  (as  in  this  Procedure),  it  should  be  heated  to  boiling  between  each 
pouring. 

2.  Owing  to  the  imperfect  washing  of  the  NH4C1  precipitate,  the 
aqueous  extract  may  contain,  besides  lead,  small  quantities  of  other 
elements.  Barium,  if  present,  would  give  a  precipitate  with  the 
K2CrO4  in  neutral  solution ;  but  this  is  prevented  by  the  addition  of 
the  2  cc.  of  HNOs.  The  use  of  much  more  HNOs  or  of  much  less 
K2CrO4  solution  would  so  increase  the  solubility  of  PbCr04  as  to 
seriously  diminish  the  delicacy  of  the  lead  test. 

Procedure  13.  —  Detection  of  Silver  and  Mercury.  —  Pour 
repeatedly  through  the  filter  containing  the  residue  insoluble 
in  hot  water  (P.  12)  a  5-10  cc.  portion  of  NH4OH  (see  Note  i). 
(Black  residue  on  the  filter,  presence  of  MERCUROUS  MERCURY.) 
Acidify  the  filtrate  with  HN03.  (White  precipitate,  presence 
of  SILVER.)  When  there  is  much  black  residue  and  little  or  no 
white  precipitate,  treat  the  residue  by  P.  14. 

Notes.  —  i.  When  two  quite  different  limiting  quantities  of  the  reagent 
are  specified  (for  example,  5-10  cc.  as  in  this  Procedure),  the  quantity 
added  should  be  adjusted  to  the  size  of  the  precipitate.  The  upper  limit 
is  so  specified  as  to  provide  for  the  presence  of  500  mg.  of  the  element  con- 
cerned. 

2.  The  black  residue  that  is  produced  by  the  action  of  NEUOH  on 
HgaCU  is  a  mixture  of  finely  divided  mercury  with  the  white  mercuric 
compound  HgClNH2.  The  reaction  is  expressed  by  the  equation : 

Hg2Cl2+2  NH4OH= HgClNH2+Hg+NH4+Cr-+  2  H2O. 
The  compound  HgClNHj  may  be  considered  to  be  a  derivative  of 


P.  14  ANALYSIS  OF  SILVER-GROUP  65 

HgCl2,  formed  by  replacing  an  atom  of  chlorine  by  the  univalent 
radical  NH2. 

3.  An  NH4OH  solution  contains  a  considerable  proportion  of  (un- 
hydrated)  NH3 ;  and  AgCl  dissolves  readily  in  it,  owing  to  the  formation 
of  a  soluble  complex  salt,  Ag(NH3)2Cl,  which  in  solution  is  largely 
ionized  into  Ag(NH3)2+  and  Cl~  ions.    This  complex  cation  has  so 
slight  a  tendency  to  dissociate  into  Ag+  and  NH3  that  the  ratio  of  its 
concentration  to  that  of  the  simple  Ag+  ion  is  about  io7  in  a  normal 
solution  of  NH4OH. 

4.  If  the  PbCl2  was  not  completely  extracted  from  the  NH4C1  pre- 
cipitate by  boiling  water  (in  P.  12),  it  is  converted  into  a  basic  salt 
(Pb(OH)Cl)  by  the  NH4OH,  and  may  pass  through  the  filter,  yielding  a 
turbid  filtrate.    This  basic  salt  will,  however,  dissolve  on  the  addition 
of  HN03. 

Procedure  14.  —  Detection  of  Silver  in  the  Presence  of  Much 
Mercury.  —  Wash  the  black  residue  undissolved  by  NH4OH 
(P.  13),  and  pour  repeatedly  through  the  filter  containing  it  a 
mixture  of  3  cc.  of  HC1  and  io  cc.  of  saturated  Br2  solution,  at 
the  same  time  rubbing  the  residue  with  a  glass  rod.  Wash  the 
filter,  and  pour  repeatedly  through  it  a  io  cc.  portion  of  NHUOH. 
Acidify  the  solution  with  HNOs.  (Yellowish-white  precipitate, 
presence  of  SILVER.) 

Notes.  —  i .  When  much  mercury  is  present  a  considerable  quantity 
of  silver  (5  mg.  or  more)  may  be  so  completely  retained  in  the  black 
residue  that  scarcely  any  test  for  silver  is  obtained  in  P.  13.  This  is 
probably  due  to  the  fact  that  the  AgCl  is  reduced  to  metallic  silver  by 
the  metallic  mercury.  When  much  mercury  is  present  it  is  therefore 
necessary  to  test  the  residue  for  silver,  as  described  in  this  Procedure. 

2.  The  Br2  converts  the  mercury  in  the  residue  into  soluble  HgBrz> 
and  the  silver  into  insoluble  AgBr.  The  HC1  dissolves  the  HgClNH, 
present  in  the  residue  with  formation  of  HgCl2. 


66         PRECIPITATION  OF  COPPER  AND  TIN  GROUPS     P.  21 
PRECIPITATION  AND  SEPARATION  OF  THE  COPPER  AND  TIN  GROUPS 


TABLE  IV.  —  PRECIPITATION  AND  SEPARATION  OF  THE  COPPER  AND 
TIN  GROUPS. 

Hydrogen  Sulfide  Precipitate: 
PbS,  Bi2S3,  CuS,  CdS. 
HgS,  AsaSs,  As2S6,  SbzSs,  SbjS5,  SnS,  SnS2. 

Treat  with  Na^S-NozSz  solution  (P.  22). 


Residue  :  PbS,  Bi2S3,  CuS,  CdS. 
See  Table  V. 

Solution:  Na2HgS2,  NasAsS^  Na3SbS4, 
Na2SnS3. 
Acidify  with  HCl  (P.  23). 

Precipitate  : 
HgS,  As2S5,  S\»S6,  SnS2,S. 
See  Table  VI. 

Filtrate  : 
NaCl. 
Reject. 

Procedure  21.  —  Precipitation  of  the  Copper  and  Tin  Groups. 
—  Dilute  to  100  cc.  the  filtrate  from  the  NH4C1  precipitate 
(P.  u)  or  the  solution  of  the  substance  in  HCl  or  H2SO4  (P.  3-9), 
which  should  contain  about  30  milliequivalents  of  HNO3  or  HCl, 
or  50  milliequivalents  of  H2SO4.  Pour  this  solution,  without 
filtering  off  any  precipitate,  into  a  conical  flask  provided  with  a 
two-hole  rubber  stopper  in  which  is  a  tube  leading  to  the  bottom 
of  the  flask.  Pass  into  the  cold  solution  through  a  gas  wash- 
bottle  a  slow  current  of  H2S,  till,  upon  closing  the  hole  in  the 
stopper  and  shaking  the  flask,  the  gas  no  longer  continues  to 
bubble  through  the  wash-bottle  into  the  solution.  Filter,  wash 
the  precipitate  with  hot  water  (see  Note  i),  and  treat  it  by  P.  22, 
after  uniting  with  it  any  further  precipitate  obtained  by  the 
H2S  treatment  described  in  the  last  paragraph  of  this  Procedure. 
Heat  the  filtrate  nearly  to  boiling  (to  70-90°),  and  pass  H2S  into 
it  at  that  temperature  for  5-10  minutes. 

In  case  there  is  no  further  precipitate,  treat  5  cc.  of  the  solu- 
tion by  P.  50  and  the  remainder  by  P.  51. 

In  case  there  is  a  further  precipitate,  add  5  cc.  of  12  n.  HCl, 
and  evaporate  the  mixture  just  to  dryness  (see  Note  2).  Then 
add  10  cc.  of  6  n.  HCl,  saturate  the  cold  solution  with  H2S,  heat 


P.  21     PRECIPITATION  OF  COPPER  AND  TIN  GROUPS         67 

it  to  70-90°,  and  pass  H2S  into  it  for  5-10  minutes.  Cool  the 
mixture,  dilute  it  to  100  cc.,  and  saturate  it  with  H2S.  Filter 
out  the  precipitate,  wash  it,  and  unite  it  with  the  first  H^S 
precipitate.  Treat  5  cc.  of  the  nitrate  by  P.  50  and  the  re- 
mainder by  P.  51. 

Notes.  —  i.  The  washing  of  precipitates  should  in  general  be  con- 
tinued until  the  wash-water  will  no  longer  give  a  test  for  any  substance 
known  to  be  present  in  the  filtrate  (for  example,  in  this  case  for  acid  with 
blue  litmus-paper  or  for  chloride  with  AgNOs).  Precipitates  which  are 
practically  insoluble  in  water  (like  all  the  sulfides  and  hydroxides  that  are 
met  with  in  this  system  of  analysis)  are  best  washed  with  nearly  boiling  water, 
as  this  runs  through  the  filter  more  rapidly  and  extracts  soluble  substances 
more  readily.  Precipitates  which  are  appreciably  soluble  should  be 
washed  with  cold  water  and  with  only  a  small  quantity  of  it.  The  proper 
method  of  washing  a  precipitate  is  to  cause  a  fine  stream  of  water  from  a 
wash-bottle  to  play  upon  the  upper  edge  of  the  filter  (and  in  larger  quantity 
on  the  three-fold  part  of  it).  The  wash-water  should  in  general  not  be 
allowed  to  run  into  the  filtrate,  so  as  not  to  dilute  it  unnecessarily. 
When,  however,  a  considerable  proportion  of  the  solution  would  be  re- 
tained in  the  filter  and  precipitate,  it  is  well  to  add  the  first  washings  to 
the  filtrate. 

2.  When  it  is  directed  to  evaporate  a  solution  almost  to  dryness  or 
just  to  dryness,  the  last  part  of  the  evaporation  should  be  carried  out  by 
keeping  the  dish  moving  over  a  small  flame  in  such  a  way  as  not  to  over- 
heat the  residue  and  as  to  avoid  bumping.     The  expression  "  almost  to 
dryness  "  implies  that  the  evaporation  is  discontinued  while  the  residue 
is  still  moist  (with  0.5-1.0  cc.  of  the  solution) ;   the  expression  "just  to 
dryness"  that  it  is  continued  till  the  residue  becomes  dry  (but  without 
any  of  it  having  been  heated  above  about  125°). 

3.  The  H+  ion  concentration  has  been  made  0.3  normal  (by  diluting  5 
cc.  of  6  n.  HNO3  or  HC1  to  100  cc.)  and  the  solution  is  saturated  with 
H2S  gas  in  the  cold,  since  under  these  conditions  there  is  a  precipitate 
with  even  i  mg.  of  cadmium,  lead,  or  tin  (the  elements  of  the  copper- 
and  tin-groups  least  readily  precipitated),  and  there  remain  in  solution 
even  300  mg.  of  zinc  (the  element  of  the  aluminum-  and  iron-groups 
most  likely  to  precipitate).     (This  statement  in  regard  to  zinc  is  true, 
however,  only  when  the  solution  contains  also  a  considerable  quantity 
of  chloride-ion,  such  as  was  added  in  P.  n,  and  when  it  is  not  allowed 
to  stand.)     Moreover,  even  when  a  small  quantity  of  any  of  the  ele- 
ments of  the  aluminum-  and  iron-groups  is  present  with  a  large  quan- 
tity of  a  copper-group  element,  the  former  is  not  carried  down  in  the 


68        PRECIPITATION  OF  COPPER  AND  TIN  GROUPS    P.  81 

H2S  precipitate  under  these  conditions  to  such  an  extent  as  to  prevent 
its  detection  in  the  filtrate,  provided  as  much  of  it  as  i  mg.,  or  in 
some  combinations  as  much  as  2  mg.,  is  present. 

4.  The  formation  of  a  white  precipitate  on  diluting  the  solution  to 
100  cc.  shows  the  presence  of  considerable  bismuth  or  antimony.    The 
precipitate",  which  consists  of  BiOCl  or  SbOCl,  need  not  be  filtered  off, 
as  these  substances  are  converted  into  sulfides  by  H2S.    The  formation, 
on  passing  in  H2S,  of  a  white  or  yellowish  precipitate  which  rapidly 
turns  black  with  more  H2S  indicates  mercury.     (The  white  compound 
is  HgCl2  •  2  HgS,  and  this  is  converted  into  black  HgS  by  the  excess 
of  H2S.)     The  formation  of  an  orange  precipitate  shows  antimony ; 
of  a  yellow  one,  cadmium,  arsenic,  or  stannic  tin.     All  the  other  sulfides 
are  black  or  brownish  black. 

5.  The  precipitation  of  bismuth  and  antimony  on  diluting  the 
filtrate  from  the  silver-group  arises  from  the  fact  that  their  normal 
salts  (BiCl3,  Bi(NO3)3,  SbCls,  etc.),  though  very  soluble  in  fairly  con- 
centrated acids,  are  hydrolyzed  (decomposed  by  water)  with  formation 
of  oxysalts  (BiOCl,  BiONOa,  SbOCl,  etc.)  which   are   only  slightly 
soluble  in  water.    Equilibrium  is  established  between  the  precipitate 
and  the  solution  in  accordance  with  the  mass-action  law,  which  requires 
that  the  concentration  of  bismuth  or  antimony  in  the  solution  increase 
rapidly  with  the  concentration  of  the  acid.     Under  the  conditions 
of  this  Procedure  bismuth  precipitates  when  more  than  50  mg.  are 
present,  and  antimony  when  more  than  15  mg.  are  present. 

6.  The  effect  of  acid  on  the  precipitation  of  the  sulfides  is  explained 
by  the  mass-action  law  and  ionic  theory  as  follows:   When  a  dilute 
solution,  whether  aqueous  or  acid,  is  saturated  at  a  definite  temperature 
with  H2S  gas  under  the  atmospheric  (or  any  definite)  pressure,  the  H2S 
present  as  such  in  the  solution  always  has  the  same  concentration. 
This  ionizes,  however,  to  a  slight  extent  into  H+  and  HS~,  and  to  a 
still  less  extent  into  2  H+  and  S=.    It  is  only  the  latter  form  of  ioniza- 
tion  that  needs  to  be  considered  here.    Now  between  the  H2S  and  its 
ions  must  be  maintained  the  equilibrium  expressed  by  the  equation 
(H+)2X(S=)  =  const.  X(H2S);    or,  since  in  this  case  (H2S)=  const.,  as 
just  stated,  it  follows  that  also  (H+)2X(S=)  =  const.     From  this  it  is 
evident  that  when  (H+)  is  increased  by  the  addition  of  acid  to  the 
solution,  (S=)  must  be  decreased  in  the  proportion  in  which  the  square 
of  (H+)  is  increased ;  thus,  if  (H+)  is  doubled,  (S=)  will  be  reduced  to 
one-fourth.    But  in  order  that  a  sulfide  —  for  example,  of  the  formula 
M-'-'-S35  — may   precipitate,    the   concentration-product    (M++)X(S=) 
must  attain  its  saturation-value.    This  value  varies,  however,  with 
the  nature  of  the  sulfide  and  with  the  temperature ;  and  therefore  the 
acid  concentration  that  will  barely  permit  of  precipitation  when  (M++) 


P.  21     PRECIPITATION  OF  COPPER  AND  TIN  GROUPS         69 

has  a  definite  value  (for  example,  i  mg.  in  100  cc.)  will  be  different  for 
different  sulfides  and  for  the  same  sulfide  at  different  temperatures. 
Thus,  if  the  elements  are  arranged  in  the  order  in  which  they  are  pre- 
cipitated from  cold  HC1  solutions  as  the  acid-concentration  is  pro- 
gressively decreased,  the  series  is  approximately  as  follows:  arsenic, 
mercury  and  copper,  antimony,  bismuth  and  stannic  tin,  cadmium, 
lead  and  stannous  tin,  zinc,  cobalt,  nickel,  iron,  manganese.  The 
acid  concentration  which  permits  precipitation  also  varies  with  the 
ionization  of  the  acid ;  thus  zinc  is  precipitated  from  a  fairly  concen- 
trated solution  of  acetic  acid,  since,  owing  to  the  slight  ionization  of 
this  acid,  the  H+  concentration  is  less  than  in  a  far  more  dilute  solution 
of  HC1.  The  three  acids,  HC1,  HN03,  and  H2SO4,  afford  another 
instance  of  the  effect  of  difference  in  ionization.  As  will  be  seen  from 
the  Table  of  Ionization  Values  in  the  Appendix,  the  first  two  of  these 
acids  are  almost  completely  ionized  at  moderate  concentrations ;  but, 
in  the  case  of  H2SO4,  while  the  first  hydrogen  is  almost  completely 
dissociated,  the  second  hydrogen  is  split  off  (from  the  ion  HSO4~)  to 
only  a  moderate  extent  (about  25%),  so  that  in  order  to  yield  a  hy- 
drogen-ion concentration  of  0.3  normal,  about  50  milliequivalents  of 
H2S04  have  to  be  present  in  100  cc.  of  solution  (instead  of  the  30 
milliequivalents  of  HC1  or  HN03). 

7.  The  solution  is  filtered,  heated  nearly  to  boiling,  and  again 
saturated  with  H2S,  in  order  to  insure  the  detection  of  arsenic;   for 
this  element,  when  present  in  the  higher  state  of  oxidation  (as  arsenic 
acid)  is  only  very  slowly  precipitated  by  H2S  in  the  cold.    At  70-90° 
the  precipitation  is  much  more  rapid,  especially  if  the  solution  has  been 
previously  saturated  with  H2S  in  the  cold.    Under  these  conditions 
even  i  mg.  of  arsenic  gives  a  distinct  precipitate  in  less  than  5  minutes. 
Continuous  treatment  with  H2S  at  70-90°  in  an  open  vessel  does  not, 
however,  completely  precipitate  a  large  quantity  of  arsenic  from  such 
a  weakly  acid  solution  even  within  an  hour.     For  this  reason,  when  a 
considerable  precipitate  forms  in  the  hot  solution,  it  is  directed  to 
evaporate  the  filtrate,  to  add  HC1  to  destroy  the  HNO3  (which  in  the 
concentrated  state  would  decompose  the  H2S),  to  dissolve  the  residue 
in  HC1,  and  to  pass  H2S  through  the  hot  solution.     From  this  concen- 
trated acid  solution  the  arsenic  precipitates  completely  in  5-10  minutes. 
The  reasons  for  this  peculiar  behavior  of  arsenic  in  the  higher  state  of 
oxidation  are  stated  in  the  following  note.    The  solution  is  finally 
diluted  and  saturated  in  the  cold  with  H2S,  since  the  other  elements 
are  not  completely  precipitated  until  the  arsenic  has  been  removed. 

8.  When  a  solution  of  H3AsO4  in  dilute  HC1  is  treated  with  H2S, 
soluble  sulf arsenic  acid  (HjAsOaS)  is  formed,  which  explains  why  the 
solution  may  absorb  much  of  the  gas  before  a  precipitate  appears. 


70        PRECIPITATION  OF  COPPER  AND  TIN  GROUPS    P.  21 

This  compound  is  decomposed  slowly  in  the  cold,  but  much  more 
rapidly  on  heating,  with  precipitation  of  a  sulfide  of  arsenic  (As2S3  and 
S2  when  excess  of  H2S  is  not  present ;  AszSs  when  the  solution  is  kept 
saturated  with  the  gas) .  This  decomposition  takes  place  more  rapidly, 
the  greater  the  concentration  of  the  acid.  In  fairly  concentrated 
HC1  solution  HjAsO4  is  also  directly  converted  by  excess  of  H2S  into 
AsiiSs-  The  slow  precipitation  of  arsenic  when  in  the  form  of  HsAsO* 
(or  HsAs03S)  is  due  to  the  extremely  small  concentration  of  arsenic- 
ion  (As+++++)  in  the  solution ;  and  the  fact  that  its  precipitation,  un- 
like that  of  the  other  elements,  is  greatly  promoted  by  a  large  HC1 
concentration  doubtless  arises  from  a  partial  conversion  of  the  HjAsO* 
into  AsCl5,  which  by  its  ionization  yields  arsenic-ion. 

9.  A  white,  finely  divided  precipitate  of  free  sulfur  will  be  formed 
if  the  solution  contains  substances  capable  of  oxidizing  H2S.    The 
most  important  of  these  likely  to  be  present  are  ferric  salts,  chromates, 
permanganates,  and  chlorates.    In  dilute  solution  the  reduction  by 
H2S  of  ferric  salts  to  ferrous  is  attended  by  a  change  in  color  from 
yellow  to  colorless;    of  chromates  to  chromic  salts,  from  orange  to 
green;    and  of  permanganates  to  manganous  salts,  from  purple  to 
colorless.    Nitric  acid,  if  it  were  fairly  concentrated,  would  also  destroy 
the  H2S;    but  at  the  concentration  in  question  (0.3  normal)  it  has 
scarcely  any  oxidizing  action  even  in  boiling  solution. 

10.  In  balancing  equations  expressing  reactions  of  oxidation  and 
reduction,  like  those  referred  to  in  the  preceding  note,  the  main  thing 
is  to  determine  the  number  of  molecules  of  the  oxidizing  and  reducing 
substances  which  react  with  one  another.     This  can  be  done  most 
simply  by  considering,  in  the  way  illustrated  by  the  following  examples, 
the  changes  which  take  place  in  the  valences  of  the  atoms  of  these 
substances.    Thus,  in  the  reduction  of  a  ferric  to  a  ferrous  salt  by 
hydrogen  sulfide,  the  iron  atom  changes  hi  valence  from  +3  to  +2,  and 
the  sulfur  atom  changes  in  valence  from  —2  (in.  H2S)  to  zero  (in  ordinary 
sulfur).     Since  the  total  change  in  the  number  of  valences  must  be 
equal  and  opposite  hi  the  two  substances,  it  is  evident  that  two  mole- 
cules of  ferric  salt  react  with  one  of  hydrogen  sulfide,  and  therefore 
that  the  equation  is : 

2  FeCl3+H2S=  2  FeCl2+S+ 2  HC1. 

In  the  reduction  of  HC103  to  HC1  by  H2S,  the  chlorine  atom  decreases 
in  valence  from  +5  to  —  i  (thus  by  six  positive  valences) ;  hence  there 
must  be  a  decrease  of  six  negative  valences  in  the  reducing  substance, 
and  that  this  may  be  the  case  three  molecules  of  H2S  are  evidently 
required.  The  equation  is  therefore : 

HC10s+3  H2S=HCl+3  8+3  H2O. 


P.  22       SEPARATION  OF  COPPER  AND  TIN  GROUPS  71 

In  cases  where  the  valence  of  an  atom  in  a  substance  is  in  doubt, 
it  can  be  found  at  once  from  the  valences  of  the  other  atoms  with  the 
aid  of  the  principle  that  in  any  compound  the  sum  of  all  the  positive 
valences  is  equal  to  the  sum  of  all  the  negative  valences;  thus  in 
chloric  acid  HClOs,  since  the  three  oxygen  atoms  have  6  negative 
valences  and  the  hydrogen  atom  has  i  positive  valence,  the  chlorine 
atom  must,  in  order  to  make  the  compound  neutral,  have  5  positive 
valences. 

Consider  as  another  example  the  reduction  of  potassium  perman- 
ganate (KMn04)  to  manganous  chloride  (MnCl2)  by  H2S  in  the  presence 
of  HCl.  The  number  of  valences  of  the  manganese  atom  is  seen  to  be 
+7  in  KMn04  (since  that  of  four  oxygen  atoms  is  —8  and  that  of  the 
potassium  atom  is  +i)  and  to  be  +2  in  MnCl2.  The  proportion  is 
therefore  2  KMnCX :  5  H2S,  and  the  reaction  is : 

2  KMnOi-l-S  H2S+6  HC1=  2  MnCl2+2  KC1+S  S+8  H2O. 

The  amount  of  acid  required  in  such  cases  can  be  seen  by  inspection, 
—  most  readily  by  noting  how  many  hydrogen  atoms  are  needed  to 
combine  with  the  oxygen  atoms  of  the  substance  undergoing  reduction. 
Thus  in  this  case  16  hydrogen  atoms  are  evidently  needed  for  this 
purpose ;  and,  since  10  are  furnished  by  the  H2S,  6  more  must  be 
supplied  by  adding  6  molecules  of  HCl  (or  an  equivalent  quantity  of 
some  other  acid). 

Procedure  22.  —  Separation  of  the  Copper-Group  from  the 
Tin-Group  by  Sodium  Sulfide.  —  Transfer  the  H2S  precipitate 
(P.  21)  to  a  casserole  (see  Note  i),  and  add  to  it  3-10  cc.  (see 
Note  2,  P.  u,  and  Note  i,  P.  13)  of  Na2S  reagent.  Cover  the 
dish,  ^nxj^jjeat  the  mixture  to  50-70°  for  3-5  minutes  with 
constant  agitatkm.  Add  5-10  cc.  of  water,  and  filter.  (Residue, 
presence  of  the  COPPER-GROUP.)  Wash  the  residue  thoroughly 
with  hot  water.  (See  Note  i,  P.  21.)  (Residue,  P.  31;  solu- 
tion, P.  23.) 

Notes. — i .  When  a  precipitate  is  to  be  transferred  to  a  casserole,  the  filter 
is  opened,  the  portions  to  which  no  precipitate  adheres  are  torn  off,  and  the 
remainder  is  laid  along  the  side  of  a  casserole;  the  solvent  is  then  poured 
over  it  and  is  swashed  to  and  fro,  the  precipitate  being  rubbed  at  the  same 
time  with  a  glass  rod,  so  as  to  remove  it  from  the  filter.  If  this  succeeds, 
the  filter  is  drawn  out  of  the  solution,  the  liquid  pressed  out  of  it  with  a 
glass  rod,  and  the  paper  thrown  away;  otherwise  the  filter  is  allowed  to 
disintegrate  and  is  filtered  out  together  with  any  residue. 


72  SEPARATION  OF  COPPER  AND  TIN  GROUPS       P.  22 

2.  The  Na2S  reagent  is  a  solution  3  normal  in  Na2S,  i  normal  in 
Na2Sj,  and  i  normal  in  NaOH.    It  is  prepared  by  dissolving  sulfur 
in  a  solution  of  Na2S  and  NaOH.    The  NaOH  serves  to  diminish  the 
hydrolysis  of  the  sulfides. 

3.  Sodium  sulfide  dissolves  the  sulfides  of  the  tin-group  because  it 
converts  them  into  soluble  salts  of  sulfo-acids  with  complex  anions. 
In  the  case  of  the  higher  sulfides  the  reactions  are  as  follows  : 

As2§6  +  3  Na+2S=  =  2  Na+3  AsS4'". 
Sb2S6  +  3  Na+2S=  =  2  Na+3  SbS4—  . 
SnS2  +  Na+2S= 


The  excess  of  sulfur  present  in  the  form  of  Na^  in  the  reagent  oxidizes 
the  lower  sulfides  (As2S3,  Sb2S3,  SnS)  to  the  same  sulfosalts  as  are  pro- 
duced by  the  action  of  Na2S  on  the  higher  sulfides.  It  will  be  seen 
that  these  sulfosalts  are  analogous  to  the  salts  of  the  familiar  oxygen 
acids  of  these  elements,  the  difference  being  that  sulfur  has  replaced 
oxygen  ;  and  they  are  so  named  as  to  indicate  this  relationship.  Thus 
the  four  sulfosalts  whose  formulas  are  given  above  are  called  sodium 
sulfarsenate,  sulfantimonate,  sulfostannate,  and  sulfomercurate. 

4.  For  separating  the  copper-group  from  the  tin-group  a  sulfide 
reagent  containing  no  Na2S2  may  be  used  in  case  any  tin  present  must 
be  in  the  higher  state  of  oxidation  as  a  result  of  the  use  of  concentrated 
HN03  (in  P.  3  or  4)  in  preparing  the  solution  of  the  original  substance. 
This  has,  however,  no  decided  advantages  ;  and  it  has  the  defect  that 
the  separation  of  a  small  quantity  of  mercury  from  a  large  quantity 
of  copper  or  cadmium  is  less  complete.    The  reagent  containing  Na2S2 
must  be  used  in  case  tin  may  be  present  in  the  lower  state  of  oxidation  ; 
for  sodium  monosulfide  does  not  dissolve  SnS. 

5.  The  behavior  of  the  various  sulfides  when  warmed  with  10  cc. 
of  the  Na2S  reagent  is  as  follows  :  Of  the  sulfides  of  the  copper-group, 
only  a  little  bismuth  and  a  little  copper  (up  to  i  mg.  of  each)  dissolve. 
Of  the  sulfides  of  the  tin-group,  more  than  500  mg.  of  any  of  them  pass 
into  solution.     Even  when  a  large  quantity  (500  mg.)  of  any  one  basic 
constituent  is  present,  the  separation  is  sharp  enough  to  enable  a  small 
quantity  (1-2  mg.)  of  any  other  basic  constituent  to  be  detected  in 
the  subsequent  analysis;   except  that  a  little  mercury  (up  to  2  mg.) 
may  remain  almost  entirely  in  the  copper-group  residue  when  a  large 
quantity  (500  mg.)  of  copper  or  cadmium  is  present. 

6.  Ammonium   monosulfide   and  disulfide  are  employed  in  most 
schemes  of  qualitative  analysis  for  the  separation  of  the  sulfides  pre- 
cipitated by  H2S  into  two  groups  ;  but  the  sodium  sulfide  reagent  here 
utilized  has  the  following  advantages.    The  quantity  of  copper  dis- 


P.  23       SEPARATION  OF  COPPER  AND  TIN  GROUPS  73 

solved  by  the  Na2S  reagent  (i  mg.  in  10  cc.)  is  much  less  than  that 
(5-10  mg.)  dissolved  by  the  ammonium  disulfide  reagent.  The  Na2S 
reagent  dissolves  mercury,  thus  separating  it  from  the  copper-group; 
while  the  ammonium  sulfide  reagents  leave  it  undissolved  hi  the  copper- 
group  residue,  with  the  disadvantage  that  the  precipitate  of  mercuric 
sulfide  may  retain  considerable  cadmium  (up  to  5  mg.),  making  its 
detection  less  delicate.  Moreover,  in  the  sodium  sulfide  separation 
the  presence  of  tin  causes  no  complications;  but  in  the  ammonium 
sulfide  process,  in  case  a  small  quantity  (2-15  mg.)  of  tin  is  present  with 
a  large  quantity  (100-500  mg.)  of  any  element  of  the  copper-group,  all 
the  tin  may  remain  in  the  undissolved  residue,  making  it  necessary  to 
provide  for  its  detection  at  two  places  in  the  scheme  of  analysis.  Fi- 
nally, the  sodium  sulfide  reagent  is  prepared  more  easily  and  in  a  more 
standard  condition.  On  the  other  hand,  the  Na2S  reagent  has  the 
disadvantages  that  it  dissolves  a  little  bismuth  (i  mg.  in  10  cc.),  while 
the  ammonium  sulfide  reagents  leave  it  entirely  undissolved,  and  that 
it  fails  to  separate  a  small  quantity  of  mercury  (up  to  2  mg.)  from  a 
large  quantity  of  copper  or  cadmium. 

Procedure  23.  —  Precipitation  of  the  Tin-Group.  —  To  the 
Na2S  solution  (P.  22)  in  a  small  flask  gradually  add  HC1  till  the 
mixture  becomes  acid  (see  Note  i),  then  add  i  cc.  more,  and 
shake  the  mixture  for  a  minute  or  two. 

In  case  the  precipitate  is  nearly  white  (see  Note  5)  (showing 
the  absence  of  the  TIN-GROUP),  reject  the  mixture. 

In  case  the  precipitate  is  black,  yellow,  or  orange  colored 
(showing  the  presence  of  the  TIN-GROUP),  filter  it  out  and  wash 
it  with  the  aid  of  suction  (see  Note  2),  finally  sucking  it  as  dry 
as  possible.  Reject  the  filtrate,  and  treat  the  precipitate  im- 
mediately by  P.  41. 

In  case  the  precipitate  is  neither  nearly  white  nor  of  a  pro- 
nounced black,  yellow,  or  orange  color  (making  doubtful  the 
presence  or  absence  of  the  TIN-GROUP),  transfer  it  to  a  casserole, 
warm  it  with  8  cc.  of  NH4OH  for  2-3  minutes  with  frequent 
agitation,  and  filter.  Treat  the  residue,  if  it  is  dark  colored  (show- 
ing the  possible  presence  of  MERCURY),  by  P.  42-43.  To  the 
filtrate  add  5  drops  of  (NH4)2S  reagent,  heat  it  to  boiling,  filter 
out  any  precipitate,  add  to  the  solution  10  cc.  of  water,  acidify 
the  mixture  with  HC1  (see  Note  i),  and.  shake  it  for  a  minute  or 


74  SEPARATION  OF  COPPER  AND  TIN  GROUPS       P.  23 

two.  If  the  precipitate  is  yellow  or  orange  colored  (showing 
the  presence  of  ARSENIC,  ANTIMONY,  or  TIN),  filter  it  out,  wash  it, 
and  treat  it  by  P.  41-47  (omitting  P.  42  for  the  separation  of 
mercury).  If  the  precipitate  is  white  (showing  the  absence  of 
ARSENIC,  ANTIMONY,  and  TIN),  reject  it. 

Notes.  —  i.  Whenever  it  is  directed  to  make  a  solution  acid  or  alkaline, 
this  should  be  done  carefully  as  follows  :  Add  from  a  graduate  somewhat 
less  acid  or  alkali  than  will  neutralize  the  alkali  or  acid  known  to  be  present 
in  the  solution.  Then  add  from  a  dropper  more  of  the  acid  or  alkali, 
10-15  drops  at  a  time,  till  a  glass  rod  dipped  in  the  solution  and  touched 
to  a  piece  of  blue  or  red  litmus  paper  placed  on  a  watch-glass  changes  the 
color  of  the  litmus  to  a  pronounced  red  or  blue  (not  to  an  intermediate 
Purple  color).  Thus  in  this  case,  if  10  cc.  of  5  n.  NaaS  reagent  were 
used  and  none  was  lost  in  the  operations,  8.3  cc.  of  6  n.  HC1  would 
be  needed  to  neutralize  it,  and  6-7  cc.  of  it  might  be  rapidly  added, 
after  which  the  remainder  would  be  added  slowly  from  a  dropper  till 
a  drop  of  the  mixture  gave  a  pure  red  color  to  litmus  paper. 

2.  In  cases  where  the  precipitate  is  voluminous,  where  the  precipitate 
must  be  washed  with  very  little  water,  or  where  (as  in  this  case)  it  must 
be  freed  as  far  as  possible  from  water,  it  is  advisable  to  filter  with  the  aid 
of  suction.     This  operation  is  carried  out  by  reinforcing  the  ordinary 
filter  with  a  small  hardened  filter  placed  below  it  in  the  funnel,  inserting 
the  funnel  in  a  rubber  stopper  in  the  neck  of  a  filter-bottle,  and  connecting 
the  side-arm  of  the  filter-bottle  to  a  suction-pump  by  means  of  a  rubber 
tube  carrying  a  screw-clamp.     The  suction  should  be  applied  very  gradu- 
ally so  as  to  avoid,  breaking  the  filter.     The  filtrate  should  be  poured  out 
of  the  filter-bottle  before  beginning  to  wash  the  precipitate. 

3.  When  the  HC1  is  added  to  the  solution  of  the  sulfosalts,  the 
corresponding  sulfoacids  which  are  liberated  decompose  immediately 
into  H2S  and  the  solid  sulfides.    These  are  now  necessarily  in  the  higher 
state  of  oxidation,  since  the  lower  sulfides,  if  originally  present,  have 
been  oxidized  by  the  Na2S2  present  in  the  Na2S  reagent.    The  fact 
that  the  sulfide  precipitates  when  the  solution  of  the  sulfosalt  is  acidified 
is  a  consequence  of  the  mass-action  law.    Thus,  the  complex  anions 
dissociate  according  to  the  equations, 

SnSf  =  SnS2+S=,  2  AsS4"-  =  As?Si+  3  S=, 
2SbS4—  =  Sb 


and  the  mass-action  law  evidently  requires  that,  in  any  solution  satu- 
rated with  the  solid  sulfide,  the  concentration  of  the  complex  anion, 
and  therefore  of  the  tin,  arsenic,  or  antimony  in  the  solution,  increase 


P.  23       SEPARATION  OF  COPPER  AND  TIN  GROUPS  75 

with  increasing  concentration  of  the  S=  ion.  Now  in  the  solution  of 
the  largely  ionized  Na2S  there  is  a  large  concentration  of  S=  ion ;  but 
when  the  solution  is  made  acid  with  HC1,  the  S=  ion  is  almost  com- 
pletely converted  by  the  relatively  large  concentration  of  the  H+  ion 
into  the  slightly  ionized  substances  HS~  and  HjS. 

4.  The  sulfide  solution  must  be  made  distinctly  acid  in  order  to 
insure  decomposition  of  the  suJitealts ;  but  a  large  excess  of  acid  must 
be  avoided  lest  SnS2  redissolve^ 

5.  When  the  Na2S  reagent  itself  is  acidified,  a  considerable  pale- 
yellow  or  grayish-white  precipitate  of  sulfur  results,  in  consequence  of 
the  decomposition  of  the  NazSz  in  the  reagent.    This  may  make  it 
doubtful  whether  a  small  quantity  of  elements  of  the  tin-group  is 
present.    In  any  such  doubtful  case  the  analyst  should  compare  the 
precipitate  obtained  with  that  produced  by  acidifying  an  equal  portion 
of  the  pure  Na2S  reagent  and  shaking  the  mixture.    If  the  conclusion 
is  still  uncertain,  the  HC1  precipitate  is  treated  by  the  last  paragraph 
of  this  Procedure.^ 

6.  The  conclusion  as  to  the  presence  of  a  small  quantity  of  the 
tin-group  becomes  more  uncertain  in  case  the  sulfide  solution  contains 
a  little  copper  or  bismuth,  whose  sulfides  are  slightly  soluble  in  the 
reagent  (as  stated  in  Note  5,  P.  22) ;  for  then  the  precipitate  may  have 
a  dark  orange  or  a  dark  yellow   color.    It   is  therefore  directed  to 
treat  the  HC1  precipitate  with  NH4OH,  so  as  to  separate  any  arsenic, 
antimony,  and  tin  from  the  sulfur,  whenever  these  elements  are  present 
in  quantity  (1-2  mg.)  not  sufficient  to  give  a  pronounced  yellow  or 
orange  color  to  the  precipitate,  and  when  not  enough  mercury  is  present 
to  give  a  distinct  black  color  to  the  precipitate.    It  may  be  mentioned, 
moreover,  that  a  mixture  of  SnS2  and  Sb2S6  does  not  always  have  a 
color  intermediate  between  the  yellow  and  orange  colors  of  the  separate 
sulfides,  but  that  it  may  be  brown  or  dark  gray. 

7.  By  the  treatment  of  the  HC1  precipitate  with  NH4OH  the  excess 
of  sulfur  and  any  HgS,  CuS,  or  Bi2S3  present  is  left  undissolved  (or  is 
reprecipitated  by  the  (NH4)2S  added  to  the  solution),  so  that  the 
second  HC1  precipitate  can  contain  only  sulfides  of  arsenic,  antimony, 
and  tin  and  a  very  little  sulfur.    As2Ss  dissolves  abundantly  in  NH4OH, 
and  Sb2S6  and  SnS2  in  moderate  quantity,  owing  to  the  formation  of  a 
mixture  of  salts  of  partially  sulfurated  acids,  such  as  HsAsOsS  and 
HjAsOjSj.    The  addition  of  (NHOzS  to  the  NH4OH  solution  and  the 
heating  serve  to  convert  these  into  salts  of  the  fully  sulfurated  acids, 
such  as  HjAsS4 ;  from  which  HC1  will  then  precipitate  the  simple  sul- 
fides much  more  completely. 


76 


ANALYSIS  OF  COPPER-GROUP 
ANALYSIS   OF   THE   COPPER-GROUP 


P.  31 


TABLE  V.  —  ANALYSIS  OF  THE  COPPER- GROUP. 


Residue  from  the  Sodium  Sulfide  Treatment :  PbS,  Bi2S3,  CuS,  CdS. 
Boil  with  HNOZ  (P.  31).    £ 

Solution :  Pb,  Bi,  Cu,  Cd  as  nitrates. 

i,  evaporate,  add  water  (P.  32). 


Precipitate  : 
PbS04. 
Dissolve  in 
NHtAc, 
add  K2CrOi 
(P.  33)^ 

Filtrate.    Add  NH*OH  (P.  34). 

Precipitate  : 
Bi(OH)3. 
Add  NatSnOz 
(P.  35)- 

Filtrate:   Cu 

To  a  small 
part  add 
HAc  and 
K4Fe(CN)6 
(P.  36}. 

NH3)4S04,  Cd(NH3)4S04. 

^  To  the  remainder 
add  HZSO*  and  Fe 
(P.  J7)- 

Yellow 
precipitate  : 
PbCr04. 

Black  residue  : 
Bi. 

Precipitate  : 
Cu. 

Solution  : 
CdSO4. 
Add  H2S. 

Red 

precipitate  : 
Cu2Fe(CN)6. 
White 
precipitate  : 
Cd2Fe(CN)6. 

Yellow 
precipitate  : 
CdS. 

Procedure  31.  —  Solution  of  the  Sulfides  in  Nitric  Acid. — 
Transfer  the  residue  from  the  Na2S  treatment  (P.  22)  to  a 
casserole,  and  add  5-15  cc.  of  3  n.  HNO3.  Stir  the  mixture, 
boil  it  gently  for  2-3  minutes,  and  filter  it.  (Residue,  see 
Note  3  ;  solution,  P.  32.) 

Notes.  —  i.  Boiling  3  n.  HN03  dissolves  the  sulfides  very  much 
more  rapidly  than  HC1  or  H2S04  of  the  same  concentration ;  for  with 
the  latter  acids  the  sulfide-ion  is  removed  from  the  solution  only  by 
combination  with  the  hydrogen-ion  and  by  the  volatization  of  the  H2S 
formed  thereby,  while  with  HN03  the  sulfide-ion  and  the  H2S  in  equi- 
librium with  it  may  also  be  destroyed  by  oxidation  to  ordinary  sulfur. 
The  oxidizing  action  of  HN03  is,  however,  slow,  unless  it  is  hot  and  at 
least  as  concentrated  as  2  normal.  Some  sulfur  is  always  oxidized 


P.  32  ANALYSIS  OF  COPPER-GROUP  77 

to  H2SO4  by  the  boiling  HN03 ;  but,  even  in  the  presence  of  much  lead, 
PbSO«  is  not  precipitated,  owing  to  its  moderate  solubility  in  HNOj. 

2.  When  much  lead,  copper,  or  bismuth  is  present  the  sulfur  formed 
may  enclose  enough  of  the  undissolved  sulfide  to  give  it  a  black  color ; 
but  the  heating  need  not  be  continued  till  the  residue  becomes  light 
colored. 

3.  In  case  it  is  desired  to  de£t  as  little  as  2  mg.  of  mercury  in  the 
presence  of  a  large  quantity  of  copper  or  cadmium,  the  residue  undis- 
solved by  HN03  should  be  treated  by  P.  43. 

Procedure  32.  —  Precipitation  of  Lead.  —  To  the  HN03  solu- 
tion (P.  31)  add  3  cc.  of  18  n.  HaSCX,  and  evaporate  in  a  casserole 
till  dense  white  fumes  of  H2SO4  begin  to  come  off,  adding  2  cc. 
more  of  18  n.  HaSCX  if  a  large  residue  separates  during  the 
evaporation.  Cool  the  mixture,  and  pour  it,  a  little  at  a  time, 
into  10  cc.  of  cold  J^&ter  in  a  test-tube,  cooling  the  tube  after 
each  addition.  Finally  rinse  out  the  casserole  with  the  same 
solution,  cool  the  mixture,  and  let  it  stand  5  minutes,  but  not 
much  longer.  (Fine  white  precipitate,  presence  of  LEAD.)  Filter, 
and  wash  the  precipitate  first  with  2  n.  HgSCX,  and  then  with 
about  5  cc.  of  water.  (Precipitate,  P.  33  ;  filtrate,  P.  34.) 

Notes.  —  i .  The  solution  is  evaporated  with  H2SO4  in  order  to 
expel  the  HNO3,  which  if  not  removed  would  dissolve  some  PbSO4 
and  diminish  the  delicacy  of  the  test  for  lead.  The  fact  that  the 
presence  of  HNO3  increases  the  solubility  of  PbSO4  in  water  is  due  to 
the  formation  by  metathesis  of  the  intermediate  ion  HSO4~,  which  has 
a  much  smaller  ionization  tendency  than  HNOs  or  H2SO4,  as  will  be 
seen  from  the  Table  of  Ionization  Values  in  the  Appendix. 

2.  A  fairly  large  quantity  of  H2S04  is  added,  so  as  to  diminish  the 
solubility  of  PbS04,  which  it  does  in  virtue  of  the  common-ion  effect 
and  so  as  to  hold  even  a  large  quantity  of  bismuth  in  solution. 

3.  The  mixture  is  allowed  to  stand  a  few  minutes  so  as  to  insure 
complete  precipitation  of  the  lead ;  for  the  solutions  of  crystalline  sub- 
stances, like  PbSO4,  tend  to  remain  supersaturated.    It  is  not  allowed 
to  stand  longer ;  for  otherwise,  when  much  bismuth  is  present,  it  may 
separate  as  a  coarsely  crystalline  precipitate  of  (BiO)2SO4,  to  such  ex- 
tent that  not  more  than  50  mg.  of  bismuth  remain  in  solution. 

4.  It  is  with  the  purpose  of  holding  a  large  quantity  of  bismuth  in 
solution  in  the  supersaturated  condition  that  special  care  is  also  taken 
to  keep  the  mixture  cool  during  the  dilution  of  the  H2SO4 ;  for  heating 
tends  to  break  up  the  state  of  saturation. 


78  ANALYSIS  OF  COPPER-GROUP  P.  84 

5.  If  in  spite  of  these  precautions  a  large  coarsely  crystalline  pre- 
cipitate separates,  it  should,  before  applying  the  confirmatory  test  for 
lead,  be  dissolved  in  5-10  cc.  of  HC1,  and  the  solution  treated  again  by 
P.  32.  The  smaller  quantity  of  bismuth  now  present  in  the  solution 
is  not  likely  to  precipitate  on  diluting  the  H2SO4  solution  with  water. 
Any  PbS04  present  is  dissolved  by  the  HC1;  and  this  acid  must  be 
all  evaporated  off  to  insure  comlfcte  reprecipitation  of  the  lead. 

Procedure  33.  —  Confirmatory  Test  for  Lead.  —  Pour  re- 
peatedly through  the  filter  containing  the  H2S04  precipitate 
(P.  32)  a  5-15  cc.  portion  of  3  n.  NKUAc  (ammonium  acetate) 
solution.  (See  Note  i,  P.  12.)  To  the  filtrate  add  2-5  drops 
of  3  n.  K2Cr04  solution  and  2-5  cc.  of  HAc  (acetic  acid).  (Yel- 
low precipitate,  presence  of  LEAD.) 

Notes.  —  i.  This  confirmatory  test  for  lead  should  not  be  omitted ; 
for  the  H2S04  precipitate  may  consist  not  only  0PbSO4  but  of  (BiO^SO* 
or  of  BaSO4,  which  last  closely  resembles  PbS04  hi  appearance. 
(BiO)2SO4  dissolves  in  NH^c  solution,  and  gives  a  yellow  precipitate 
on  adding  K2CrO4;  but  this  precipitate,  unlike  PbCr04,  dissolves 
readily  in  HAc.  BaS04  is  not  dissolved  by  NH^c  solution. 

2.  The  solubility  of  PbS04  in  NH^Ac  solution  depends  on  the  forma- 
tion by  metathesis  of  unionized  PbAc2,  this  salt  being  much  less  ionized 
than  most  other  salts  of  the  same  valence-type.  (See  the  Table  of 
lonization  Values  in  the  Appendix.)  On  the  addition  of  K2CrO4  to 
this  solution  the  much  less  soluble  PbCrO4  is  precipitated. 

Procedure  34.  —  Precipitation  of  Bismuth.  —  Make  the  H2S04 
solution  (P.  32)  alkaline  with  NH4OH  (see  Note  i).  (White 
precipitate,  presence  of  BISMUTH;  blue  solution,  presence  of 
COPPER.)  Filter,  and  wash  the  precipitate  thoroughly  (see 
Note  i,  P.  21).  (Precipitate,  P.  35 ;  filtrate,  P.  36  and  37.) 

Notes.  —  i.  Whenever  a  solution  is  to  be  made  alkaline  with  NHtOH, 
the  reagent  may  be  added,  first  in  quantity  nearly  equivalent  to  the  acid 
known  to  be  present,  then  10-15  drops  at  a  time,  till  a  distinct  odor  of 
NHtOH  is  perceptible  after  shaking  the  mixture  so  as  to  make  sure  that 
none  of  the  reagent  is  left  on  the  sides  of  the  flask. 

2.  The  precipitate  produced  by  NH4OH  may  consist  of  Fe(OH)»  or 
of  other  hydroxides  of  the  iron-group,  if  these  elements  were  not  re- 
moved from  the  H2S  precipitate  by  thorough  washing.  The  formation 
of  a  small  precipitate  is,  therefore,  not  a  sufficient  proof  of  the  presence 
of  bismuth ;  and  the  confirmatory  test  of  P.  35  must  be  applied. 


P.  S6  ANALYSIS  OF  COPPER-GROUP  79 

3.  Cd(OH)2  or  Cu(OH)2,  though  only  very  slightly  soluble  in  water, 
dissolves  in  NH4OH  owing  to  the  combination  of  the  Cd++  or  Cu++ 
ion  with  NHS,  forming  the  complex  cation  Cd(NH3)4++  or  Cu(NHj)4++. 
These  complex  cations  have  an  extremely  small  ionization  tendency; 
thus  in  a  normal  NH4OH  solution  the  ratio  of  the  concentration  of  the 
complex  cadmium  ion  to  the  simple  cadmium  ion  is  about  io7. 

Procedure  35.  —  Confirmatory  Test  for  Bismuth.  —  Pour 
through  the  filter  containing  the  well-washed  NH^OH  pre- 
cipitate (P.  34)  a  cold  freshly  prepared  solution  of  Na2SnO2 
(sodium  stannite)  (see  Note  i).  (Immediate  blackening  of  the 
residue,  presence  of  BISMUTH.) 

Notes.  —  i.  The  solution  of  sodium  stannite  (Na2SnO2)  is  prepared 
when  needed  by  adding  NaOH  solution,  a  few  drops  at  a  time,  to  8-10 
drops  of  SnCU  reagent  diluted  with  3  cc.  of  water  (the  mixture  being 
cooled  in  running  water  after  each  addition),  till  the  large  precipitate  of 
Sn(OH)2  first  formed  is  dissolved  and  a  clear  or  slightly  turbid  liquid 
results.  The  solution  must  be  kept  cold  while  it  is  being  prepared, 
and  it  must  be  freshly  prepared,  because  the  stannite  decomposes 
spontaneously  into  stannate  (Na2SnO»)  and  metallic  Sn,  and  because 
it  oxidizes  in  contact  with  air  to  sodium  stannate.  SnO2H2  is  an  ex- 
ample of  a  so-called  amphoteric  substance  —  one  which  acts  either  as  a 
base  or  an  acid,  as  is  shown  by  its  solubility  in  both  acids  and  alkalies. 

2.  The  final  test  with  Na2SnO2  depends  on  the  reduction  of  Bi(OH)s 
to  black  metallic  Bi.  The  test  is  an  extremely  delicate  one.  The  other 
reducible  substances,  like  HSbOs,  Fe(OH)3,  Pb(OH)2,  or  Cu(OH)2,  that 
might  possibly  be  present  in  the  NH4OH  precipitate  are  not  reduced 
by  short  contact  with  Na2SnO2  solution  in  the  cold.  Mercury,  if 
present,  would  cause  blackening ;  but  it  is  not  precipitated  by  NH4OH 
in  the  presence  of  ammonium  salt. 

Procedure  36.  —  Confirmatory  Test  for  Copper.  —  Acidify 
one-fourth  of  the  NH4OH  solution  (P.  34)  with  HAc,  add  from 
a  dropper  one  drop  of  KjFe(CN)6  solution,  and  let  the  mixture 
stand  2-3  minutes.  Then  add  3  cc.  more  of  K4Fe(CN)6  solu- 
tion. (Red  precipitate,  presence  of  COPPER.) 

Notes.  —  i.  The  confirmatory  test  for  copper  is  more  delicate  than 
the  formation  of  a  blue  color  with  NH4OH  (P.  34).  It  should,  there- 
fore, be  tried  even  when  the  NH4OH  solution  is  colorless.  Cadmium 
is  also  precipitated  by  K4Fe(CN)6;  but  the  precipitate  is  white,  and 
does  not  prevent  the  pink  color  of  the  copper  compound  from  being 


80  ANALYSIS  OF  COPPER-GROUP  P.  87 

detected,  provided  only  a  small  quantity  of  K4Fe(CN)$  is  added,  and 
the  mixture  is  allowed  to  stand ;  for  the  copper  salt,  owing  to  its  smaller 
solubility,  is  precipitated  immediately  or  formed  rapidly  by  metathesis 
from  the  precipitated  Cd2Fe(CN)6. 

2.  Nickel  yields  a  light  blue  solution  with  excess  of  NH4OH ;  but, 
if  it  be  present,  owing  to  incomplete  washing  of  the  H2S  precipitate, 
it  will  be  distinguished  from  copper  by  giving  a  green  precipitate  with 
K4Fe(CN),. 

Procedure  37.  —  Detection  of  Cadmium.  —  To  the  remainder 
of  the  NH4OH  solution  (P.  34)  add  H2SO4,  i  cc.  at  a  time,  till 
the  solution  reddens  litmus  paper,  and  then  5  cc.  more.     Heat 
the  mixture  to  50-60°,  add  to  it  about  i  cc.  of  iron  powder  (see 
Note  i),  and  shake  the  mixture  gently  for  about  two  minutes. 
Pour  it  through  a  filter ;  add  to  the  filtrate  20  cc.  of  water  (and,  if 
it  is  turbid,  a  few  drops  of  H2SO4) ;  and  saturate  it  in  a  flask  im- 
mediately with  H2S.     (Yellow  precipitate,  presence  of  CADMIUM.) 
Notes.  —  i.   In  case  copper  is  entirely  absent  (as  shown  by  P.  36), 
the  Fe  powder  need  not  be  added,  and  the  NH4OH  solution  may  be 
slightly  acidified  with  HaSC^  and  saturated  directly  with  H2S. 

2.  The  rate  at  which  the  copper  is  precipitated  by  the  iron,  like 
that  of  all  reactions  between  solid  substances  and  solutions,  increases 
with  the  temperature  and  with  the  surface  of  contact  between  the  solid 
and  the  solution.    The  copper  is  completely  precipitated  within  two 
minutes  when  i  cc.  of  iron  in  finely  powdered  form  is  used,  and  when 
the  warm  solution  is  steadily  shaken  with  it. 

3.  The  filtrate  from  the  Fe  treatment  is  saturated  with  H2S  im- 
mediately, since  otherwise  the  FeSO4  present  will  be  oxidized  by  the 
air  to  the  ferric  state,  and  produce  with  the  H2S  a  precipitate  of  sulfur. 

4.  In  case  H2S  produces  a  black  precipitate,  owing  to  the  incomplete 
removal  of  the  copper  by  the  Fe  powder  or  to  the  presence  of  mercury 
(in  case  Fe  powder  was  not  added),  heat  it  with  5-10  cc.  of  3  n.  HNOS, 
evaporate  the  solution  with  2  cc.  of  18  n.  H2S04  till  dense  white  fumes 
appear,  dilute  the  solution  with  15  cc.  of  water,  treat  it  with  Fe  powder 
as  hi  P.  37,  filter,  and  saturate  the  filtrate  with  H2S.    By  this  treatment 
any  copper  or  mercury  is  removed ;  and  a  yellow  precipitate  will  then 
be  produced  by  H2S  if  cadmium  is  present. 

5.  In  case  a  yellow  precipitate  is  produced  by  H2S,  and  the  sub- 
stance contains  much  arsenic,  antimony,  or  tin  (as  shown  by  P.  44-47), 
it  is  well  to  prove  that  the  H2S  precipitate  does  not  arise  from  one  of 
these  elements  by  washing  it  thoroughly  with  hot  water,  and  pouring 
repeatedly  through  the  filter  containing  it  a  10  cc.  portion  of  hot 


P.  37  ANALYSIS  OF  COPPER-GROUP  81 

NH4OH.    This  dissolves  AszSs,  Sb2SB,  and  SnSj ;  but  leaves  CdS  as  a 
yellow  residue  on  the  filter. 

6.  The  principles  determining  whether  a  metal  will  precipitate 
another  metal  from  solutions  of  its  salts  are  as  follows.    Of  two  given 
elements  that  one  will  precipitate  the  other  whose  "  reduction-potential" 
is  the  greater.    The  value  of  the  reduction-potential  of  an  element  is 
determined  by  a  constant  characteristic  of  the  metal  and  by  the  con- 
centration of  its  ion  in  the  solution.    The  constants  (expressed  in  volts 
and  referred  to  that  of  hydrogen  gas  against  hydrogen-ion  taken  as 
zero)  characteristic  of  the  more  important  elements  are  given  in  the 
Table  in  the  Appendix.     These  characteristic  constants,  which  are 
called  the  specific  reduction-potentials  of  the  elements,  are  so  evaluated 
as  to  represent  the  actual  reduction-potentials  of  the  respective  metals 
when  the  concentration  of  their  ions  is  i  formal.    Thus  0.13  volt  is 
the  reduction-potential  of  lead  when  the  Pb++  ion  in  the  solution  has 
a  concentration   of  i  formal.    The  actual  reduction-potential  of  a 
metallic  element  becomes  greater  by  a  definite  amount  for  each  ten- 
fold decrease  in  its  ion-concentration ;    namely,  by  0.06  volt  if  the 
ion  is  univalent,  by  0.03  volt  if  it  is  bivalent,  and  by  0.02  volt  if  it  is 
trivalent.    Thus  the  reduction-potential  of  lead  is  0.19  volt  when  the 
Pb++  ion  concentration  is  o.oi  formal. 

7.  These  principles  may  be  applied  to  the  separation  of  copper  and 
cadmium  by  means  of  iron  as  follows.    Referring  to  the  table  we  see 
that  the  specific  reduction-potentials  of  iron  and  copper  are  +0.43 
and  —0.34  volt,  respectively.    Owing  to  the  fact  that  the  iron  rapidly 
dissolves  in  the  H2S04  solution,  we  may  as  a  rough  approximation  con- 
sider that  the  ferrous-ion  concentration  is  i  formal,  and  therefore  that 
the  actual  reduction-potential  of  the  iron  is  equal  to  its  specific  re- 
duction-potential, +0.43  volt.    The  reduction-potential  of  the  copper 
will  increase  from  its  specific  value  —0.34  by  0.03  volt  each  time  the 
concentration  of  the  copper-ion  is  decreased  ten-fold ;  thus  it  will  be 
—  0.31  when  the  copper-ion  is  o.i  formal,  —0.28  when  it  is  o.oi  formal, 
—0.25  when  it  is  o.ooi  formal,  etc.    Evidently,  it  will  continue  to  be 
less  than  that  of  the  iron  (+0.43)  until  the  copper-ion  falls  to  the  ex- 
tremely low  concentration  of  icr26  formal.    Hence  substantially  all 
the  copper  will  be  precipitated.     On  the  other  hand,  in  case  of  the 
cadmium,   whose   specific   reduction-potential   is    +0.40,    its   actual 
reduction-potential  will  become  equal  to  that  of  the  iron  when  its 
concentration  becomes  o.i  formal.     Since  even  500  mg.  of  cadmium  in 
a  volume  of  40  cc.  corresponds  to  a  cadmium-ion  concentration  some- 
what less  than  o.i  formal,  no  cadmium  will  be  precipitated  by  the  iron 
under  the  actual  conditions. 


82 


ANALYSIS  OF  TIN-GROUP 


P.  41 


ANALYSIS    OF   THE   TIN-GROUP 


TABLE  VI.  —  ANALYSIS  OF  THE  TIN-GROUP. 


Precipitate  from  Sodium  Sulfide  Solution : 
S,  HgS,  As2S5,  Sb2S6,  SnS2. 
Heat  with  12  n.  HCl  (P.  41). 


Residue:  S,  HgS,  As2SB. 
Add  NHtOH  (P.  42). 


Solution:  SbCl3,  H2SnCl6. 
Dilute,  heat,  pass  in  HZS  (P.  45). 


Residue  : 
S,  HgS. 
Add  HCl 
and  KCIO3 
(P-  43). 

Solution. 
Evaporate, 
add  HN03, 
then  NH  4  OH 
(P.  44). 

Orange  precipitate: 
Sb2S3. 
Dissolve  in  HCl, 
addSn  (P.  46). 

Solution:  H2SnCl6. 
Partly  neutralize, 
pass  in  H^S  (f.  47)  > 

Precipitate:  SnS2. 
Evaporate  without 
filtering  (P.  47). 

Solution  : 
HgCl2. 
Add  SnCl2. 

Solution  : 

(NH4)3As04. 
AddMg(N03)2. 

Black  deposit  :  Sb. 
Add  NaOBr. 

Solution:  H2SnCl6. 
Boil  with  Sb. 

Black  deposit  :  Sb. 

Precipitate  : 
Hg2Cl2  or  Hg. 

Precipitate  : 
MgNH4AsO4. 
Add  AgN03. 

Solution  :  SnCl2. 
Add  HgCk. 

Red  residue  : 
AgsAsCU. 

Precipitate:  Hg2Q2. 

Procedure  41.  —  Separation  of  Arsenic  and  Mercury  from 
Antimony  and  Tin.  —  Transfer  the  precipitated  sulfides  dried 
by  suction  (P.  23)  to  a  test-tube,  add  from  a  small  graduate 
exactly  10  cc.  of  12  n.  HCl,  place  the  test-tube  in  a  small  beaker 
of  water,  heat  the  water  till  the  contents  of  the  tube  begin  to 
bubble,  and  keep  the  water  for  ten  minutes  at  a  temperature 
which  causes  only  slight  bubbling  in  the  tube,  stirring  its  con- 
tents from  time  to  time.  Then  pass  a  slow  current  of  H2S  into 
the  tube  (still  kept  in  the  hot  water-bath)  for  one  minute.  Add 
gradually  with  stirring  3  cc.  of  water,  and  filter  the  hot  mixture 
with  the  aid  of  suction  through  a  dry  filter  or  one  wet  with  6  n. 
HCl.  Remove  the  filtrate,  and  wash  the  residue  first  with  6  n. 
HCl,  then  with  hot  water.  (Residue,  P.  42  ;  solution,  P.  45.) 


P.  42  ANALYSIS  OF  TIN-GROUP  83 

Notes.  —  i.  If  a  much  weaker  HC1  solution  than  the  acid  of  12 
normal  concentration  is  used,  or  if  the  acid  becomes  diluted  by  an  un- 
necessary quantity  of  water  left  in  the  precipitate,  much  Sb2S5  will 
be  left  undissolved.  Even  with  the  strong  acid  some  Sb2S6  may  re- 
main undissolved,  especially  when  a  large  quantity  is  present,  in  which 
case  the  residue  if  small  in  amount  will  have  an  orange  color.  This 
small  quantity  of  Sb2S6  would  be  only  very  slowly  removed  by  further 
treatments  with  HC1;  it  does  not,  however,  interfere  with  the  sub- 
sequent tests  for  arsenic.  Moreover,  when  only  a  small  quantity  of 
Sb2S&  is  originally  present,  a  large  proportion  of  it  is  extracted,  so  that 
it  will  not  escape  detection.  Sb2Ss  dissolves  with  formation  of  SbClj, 
the  element  being  reduced  from  the  antimonic  state  by  the  H2S  with 
liberation  of  sulfur;  SnS2  dissolves  with  formation  of  SnCL»,  which 
unites  with  the  excess  of  HC1  to  form  H2SnClg. 

2.  If  the  solution  be  heated  so  that  only  slight  bubbling  occurs 
during  the  treatment  with  HC1,  the  amount  of  As2Ss  which  dissolves 
in  ten  minutes  is  insignificant.     But  this  is  no  longer  true  if  the  solu- 
tion be  allowed  to  boil  rapidly ;  for  the  boiling  expels  from  the  solution 
the  H2S  liberated  from  the  other  sulfides  or  by  slight  decomposition 
of  the  As2S5  itself,  and  thus  enables  the  decomposition  of  the  latter  to 
proceed  further.    As  a  precaution,  the  mixture  is  finally  saturated 
with  H2S  to  reprecipitate  any  arsenic  or  mercury  that  may  have  passed 
into  solution ;  for,  if  not  so  removed,  it  would  come  down  later  with 
the  antimony  (in  P.  45),  and  might  be  mistaken  for  it. 

3.  About  3  cc.  of  water  are  gradually  added  to  the  HC1  solution  to 
enable  it  to  be  filtered.    If  more  is  added,  Sb2Ss  may  precipitate.    If 
this  happens  after  the  filtration,  it  does  no  harm. 

4.  Care  must  be  taken  to  follow  closely  the  directions  in  regard  to 
the  quantity  of  HC1  used  and  to  avoid  any  loss  of  the  solution  in  the 
filtration ;  for  the  subsequent  separation  of  antimony  and  tin  (in  P.  45) 
depends  upon  a  proper  concentration  of  the  acid. 

5.  The  greater  part  of  any  CuS  and  Bi2S3  present  is  dissolved  by 
the  HC1.     The  copper  will  be  precipitated  later  with  the  antimony  (in 
P.  45),  and  the  bismuth  with  the  tin  (in  P.  47).    A  little  copper  remains 
with  the  As2S5,  but  this  does  not  interfere  with  the  tests  for  arsenic. 

Procedure  42.  —  Separation  of  Arsenic  from  Mercury.  — 
Transfer  to  a  casserole  (see  Note  i,  P.  22)  the  residue  undis- 
solved by  HC1  (P.  41).  Warm  it  with  5-15  cc.  of  NH4OH  for 
2-3  minutes  with  frequent  agitation,  and  filter  the  mixture. 
(Dark  colored  residue,  presence  of  MERCURY.)  Wash  the  residue. 
(Residue,  P.  43  ;  solution,  P.  44.) 


84  ANALYSIS  OF  TIN-GROUP  P.  43 

Notes.  —  i.  By  this  treatment  with  NH4OH  i  mg.  of  arsenic  can 
be  extracted  from  a  residue  containing  even  500  mg.  of  mercury.  But 
when  the  residue  consists  only  of  sulfur  and  1-3  mg.  of  arsenic,  most 
of  the  arsenic  may  be  left  with  the  sulfur,  mechanically  enclosed  in 
it.  Enough  arsenic  is,  however,  usually  extracted  to  enable  it  to  be 
detected  in  P.  44,  provided  care  be  taken  to  use  in  that  Procedure  the 
minimum  quantities  of  reagent. 

2.  As  to  the  chemical  action  of  NH4OH  on  AszSs,  see  Note  7,  P.  23. 

Procedure  43.  —  Detection  of  Mercury.  —  Transfer  the  residue 
undissolved  by  NH4OH  (P.  42)  to  a  casserole,  add  3-8  cc.  of 
HC1,  heat  the  mixture  nearly  to  boiling,  and  add  to  it  powdered 
KC103,  a  little  at  a  time,  till  the  black  residue  disappears.  Boil 
the  solution  till  it  no  longer  smells  of  chlorine,  replacing  the 
acid  which  evaporates.  Add  to  it  5-10  cc.  of  water,  and  filter 
out  the  residue.  To  the  filtrate  add  from  a  dropper  1-2  drops 
of  SnCl2  solution,  then  2-5  cc.  more.  (White  precipitate  turn- 
ing gray,  or  gray  precipitate,  presence  of  MERCURY.) 

Notes.  —  i.  A  mixture  of  HC1  and  KC103  is  used  to  dissolve  the 
HgS,  since  it  is  one  of  the  few  sulfides  that  is  very  slowly  attacked  by 
HC1  or  HNO3  alone.  It  can  most  easily  be  brought  into  solution  with 
the  aid  of  free  C12 ;  and  a  mixture  of  HC1  and  HNO3,  which  by  their 
action  upon  one  another  produce  Cl2,  might  be  used.  But  the  KClOa 
has  the  advantage  that  an  unnecessary  excess  can  be  easily  avoided, 
and  any  free  C12  in  the  solution  can  be  quickly  expelled  by  boiling  the 
solution.  The  main  product  of  the  reaction  between  KClOs  and 
HC1  is  C12 ;  but  some  C102  is  also  formed,  which  gives  the  yellow  color 
to  the  solution. 

2.  That  HgS,  unlike  most  other  sulfides,  does  not  dissolve  readily 
in  even  fairly  concentrated  HC1  or  HNOs  is  doubtless  due  to  the  much 
smaller  concentration  of  its  ions  in  its  saturated  solution  and  to  the 
fact  that  at  this  small  concentration  sulfide-ion  (or  the  H2S  in  equi- 
librium with  it  at  a  correspondingly  small  concentration)  is  oxidized 
only  very  slowly  even  by  the  HNOs.     HgS  is,  however,  readily  dis- 
solved by  C12,  a  more  vigorous  oxidizing  agent,  since  it  reacts  rapidly 
with  sulfide-ion  (or  with  H2S)  even  when  its  concentration  is  very  small. 

3.  Only  one  or  two  drops  of  SnCl2  solution  are  added  at  first,  so  as 
to  cause  the  precipitation  of  white  Hg2Cl2.    An  excess  of  SnCl2  is 
then  added,  so  as  to  convert  this  white  precipitate  into  a  fine  gray 
precipitate  of  metallic  Hg,  this  darkening  being  especially  characteristic 
of  mercury.    Moreover,  if  the  C12  was  not  completely  expelled  from 


P.  44  ANALYSIS  OF  TIN-GROUP  85 

the  solution,  it  will  oxidize  the  SnClz  contained  hi  the  first  drop  or  two 
of  solution  added. 

4.  In  case  a  large  quantity  (200-500  mg.)  of  copper  or  cadmium  is 
present,  mercury  in  quantities  up  to  2-3  mg.  may  escape  detection, 
owing  to  its  being  retained  in  the  copper-group  residue,  as  stated  in 
Note  5,  P.  22.  In  such  a  case,  the  residue  left  undissolved  by  HNOa 
in  P.  31  should  be  tested  for  mercury  by  treating  it  by  this  Procedure. 

Procedure  44.  —  Detection  of  Arsenic.  —  Evaporate  the 
NH4OH  solution  (P.  42)  almost  to  dryness  (see  Note  2,  P.  21). 
Add  2-5  cc.  of  HNO3,  and  heat  the  mixture  nearly  to  boiling 
till  the  residue  dissolves  or  only  sulfur  remains.  Evaporate 
the  mixture  almost  to  dryness,  add  1-3  cc.  of  water  and  1-3  cc. 
of  NH4OH,  filter  out  any  residue,  add  to  the  solution  in  a  test- 
tube  3-10  cc.  of  Mg(NO3)2  reagent,  shake  the  mixture,  and  let 
it  stand  10-15  minutes  with  occasional  shaking.  (White  crystal- 
line precipitate,  presence  of  ARSENIC.) 

Filter,  wash  the  precipitate  with  2-3  cc.  of  water,  and  pour 
over  it  on  the  filter  i  cc.  of  AgN03  solution  to  which  6-8  drops  of 
HAc  have  been  added.  (Dark  red  residue,  presence  of  ARSENIC.) 

Notes.  —  i.  After  the  NH4OH  is  expelled  by  the  evaporation,  the 
As2S6  (together  with  any  Sb2S5  present)  is  thrown  down  as  a  yellow 
or  orange-colored  precipitate.  This  furnishes  a  fairly  delicate  indica- 
tion of  the  presence  of  arsenic,  except  that  any  antimony  left  in  the 
residue  undissolved  by  HC1  will  also  give  an  orange  precipitate.  The 
precipitate,  without  filtering  it  off,  is  redissolved  by  HNO3,  which  has 
a  sufficiently  strong  oxidizing  action  to  convert  As2S6  into  HjAsO4. 

2.  The  precipitate  of  Mg(NH4)AsO4,  because  of  its  tendency  to 
hydrolyze  into  NH4OH  and  Mg++HAs04~,  is  more  soluble  in  water 
than  in  the  NH4OH  solution.     Its  solubility  is  also  diminished  by  the 
excess  of  Mg(N03)a  reagent,  in  virtue  of  the  common-ion  effect.     Since 
the  precipitate,  like  other  crystalline  substances,  tends  to  form  a  super- 
saturated solution,  the  mixture  is  allowed  to  stand  with  occasional 
shaking. 

3.  The  reagent  used  for  the  precipitation  is  a  solution  i  n.  hi 
Mg(N03)2,  3  n.  in  NH4N03,  and  0.2  n.  in  NH4OH.    The  NHJTO,,  by 
reducing  the  hydroxide-ion  concentration,  prevents  the  precipitation 
of  Mg(OH)2  by  the  NH4OH.    A  little  NH4OH  is  added  to  the  reagent 
to  remove  any  iron,  aluminum,  or  other  impurity  that  may  be  present 
in  the  magnesium  or  ammonium  salt.    The  nitrates  are  used,  rather 


86  ANALYSIS  OF  TIN-GROUP  P.  45 

than  the  chlorides,  so  as  not  to  produce  a  precipitate  of   AgCl  hi  the 
confirmatory  test  with  AgNO3. 

4.  The  AgNOs  added  in  the  confirmatory  test  converts  the 
Mg(NH4)  As04  into  dark  red  AgsAs04,  since  the  latter  compound  has  a 
much  smaller  solubility-product.  HAc  is  added  to  neutralize  the 
NH4OH;  for  otherwise  this  might  prevent  the  precipitation  of  the 
Ag3AsO4,  by  forming  the  complex  silver-ammonia  ion.  The  test  serves 
to  distinguish  Mg(NH4)AsO4  from  Mg(OH)2  or  any  other  hydroxide 
that  might  be  precipitated  by  NH4OH. 

Procedure  45.  —  Separation  of  Antimony  and  Tin.  —  Dilute 
the  solution  from  the  HC1  treatment  of  the  sulfides  (P.  41)  with 
water  to  a  volume  of  just  55  cc.,  transfer  it  to  a  flask  placed 
in  a  400  cc.  beaker  of  boiling  water,  and  after  the  solution  has 
become  hot  pass  into  it  a  moderate  current  of  H2S  gas  for  8-10 
minutes  but  not  longer,  keeping  the  water  in  the  beaker  gently 
boiling.  (Orange-red  precipitate,  presence  of  ANTIMONY.)  Filter 
while  hot,  using  suction  if  the  precipitate  is  large,  and  wash  the 
precipitate  with  hot  water.  (Precipitate,  P.  46 ;  filtrate,  P.  47.) 

Notes.  —  i.  By  following  carefully  the  directions  given  in  P.  41 
and  in  this  Procedure,  a  good  separation  of  antimony  and  tin  may  be 
obtained;  thus,  when  only  i  mg.  of  antimony  is  present  it  is  pre- 
cipitated, while  (stannic)  tin  gives  no  precipitate,  unless  400-500  mg. 
are  present.  If,  however,  the  HC1  solution  be  more  concentrated,  a 
small  quantity  of  antimony  will  escape  detection.  On  the  other  hand, 
if  the  HC1  solution  be  more  dilute,  or  if  it  be  not  kept  hot,  much  SnS» 
may  precipitate  when  a  large  amount  of  tin  is  present.  When  SnSj  is 
mixed  with  a  little  Sb2Ss  a  brown  precipitate  results. 

2.  If  copper  be  present  hi  the  substance,  a  little  CuS  may  separate 
at  this  point  as  a  black  precipitate. 

3.  Owing  to  the  possibility  that  substances  other  than  antimony 
may  be  precipitated  by  H2S  in  this  Procedure,  the  confirmatory  test 
described  in  P.  46  must  always  be  applied  to  any  precipitate  obtained. 
The  quantity  of  antimony  present  can,  however,  usually  be  better 
estimated  from  the  size  of  the  t^S  precipitate  than  from  that  of  the 
black  deposit  obtained  in  the  confirmatory  test. 

Procedure  46.  —  Confirmatory  Test  for  Antimony.  —  Transfer 
the  H2S  precipitate  (P.  45)  to  a  small  casserole,  heat  it  with 
2-5  cc.  of  1 2  n.  HC1,  add  2-5  cc.  of  water,  filter,  and  evaporate 
the  filtrate  to  about  2  cc.  Cool  the  solution,  and  place  in  it  a 


P.  47  ANALYSIS  OF  TIN-GROUP  87 

flat  piece  of  bright  mossy  tin.  After  5-10  minutes  pour  off  the 
liquid,  wash  the  residue  carefully  with  water  by  decantation, 
and  pour  over  it  2  cc.  of  fresh  NaOBr  (sodium  hypobromite) 
reagent  (see  Note  3).  (Black  residue  remaining  undissolved, 
presence  of  ANTIMONY.) 

Notes.  —  i.  Tin,  rather  than  a  more  reducing  metal  like  zinc,  is 
used  in  precipitating  the  antimony,  since  such  a  metal  would  also 
precipitate  tin  from  the  solution.  The  small  quantity  of  copper  that 
may  be  present  is  also  precipitated  by  the  tin ;  but  antimony  is  readily 
distinguished  by  its  coal-black  color  (given  even  by  0.1-0.2  mg.)  from 
a  red  or  brownish-black  one  of  copper.  Arsenic,  if  present,  is  also 
precipitated  on  the  tin  or  in  the  solution ;  but  it  is  distinguished  from 
antimony  by  its  behavior  with  NaOBr  solution.  These  facts  as  to  the 
precipitation  of  the  metals  are  explained  by  the  values  of  their  specific 
reduction-potentials  (given  in  the  Table  in  the  Appendix) .  These  differ 
so  greatly  from  that  of  tin  as  to  make  consideration  of  the  ion-concen- 
trations, which  are  complicated  in  the  cases  of  antimony  and  arsenic, 
of  secondary  importance. 

2.  The  treatment  with  NaOBr  solution  serves  to  prove  that  the 
black  precipitate  is  not  arsenic ;   for  this  element  is  readily  converted 
by  this  reagent  into  arsenate,  while  antimony  (as  well  as  copper)  is 
not  attacked  by  it. 

3.  Since  hypobromite  solution  rapidly  decomposes,  with  formation 
of  bromate  and  bromide,  the  reagent  should  be  prepared  when  needed 
by  adding  to  2  cc.  of  saturated  Br2  solution,  NaOH  solution,  drop  by 
drop,  till  the  solution  becomes  colorless  or  yellow,  and  then  adding  as 
many  drops  more  of  NaOH  solution. 

Procedure  47.  —  Detection  of  Tin.  —  To  the  nitrate  from  the 
H2S  precipitate  (P.  45)  add  just  4  cc.  of  15  n.  NH4OH,  cool  the 
mixture,  saturate  it  with  H2S,  cork  the  flask,  and  let  it  stand  for 
10  minutes.  (Yellowish  turbidity  or  yellow  flocculent  precipi- 
tate, presence  of  TIN.) 

In  case  H2S  has  produced  a  precipitate,  evaporate  the  mixture 
(without  filtering  it)  in  a  casserole  to  15-20  cc.,  or  further,  if 
the  precipitate  has  not  dissolved.  Add  |  cc.  of  powdered  anti- 
mony, and  boil  the  mixture  gently  for  2-3  minutes.  Filter,  and 
to  the  filtrate  add  at  once  2-3  cc.  of  HC1  and  10  cc.  of  0.2  n. 
HgCl2  solution.  (White  or  gray  precipitate,  presence  of  TIN.) 


88  ANALYSIS  OF  TIN -GROUP  P.  47 

Notes.  —  i.  The  acid  present  is  partially  neutralized  with  NH4OH 
and  the  solution  is  cooled,  in  order  to  diminish  the  solubility  of  SnSi- 
Time  is  allowed  for  the  precipitation,  because  stannic  tin  reacts  more 
slowly  with  H2S  than  most  of  the  other  elements,  doubtless  because  the 
tin  exists  mainly  in  the  HC1  solution  as  H+2SnCl<r,  and  not  as  Sn++++ 
ion.  When  only  a  small  quantity  (0.5  to  2  mg.)  of  tin  is  present,  the 
precipitate  of  SnS2  produces  in  the  solution  a  yellowish  translucent 
turbidity,  which  is  readily  distinguished  from  the  trace  of  finely  divided 
sulfur  which  may  separate. 

2.  In  the  confirmatory  test  the  precipitate  of  SnS2  is  not  filtered  off, 
but  is  dissolved  by  concentrating  the  acid  by  evaporation,  since  it 
clogs  the  filter  and  also  tends  to  pass  through  it.    The  HgCl2  reagent  is 
added  to  the  solution  as  soon  as  it  is  filtered  from  the  antimony,  since 
SnCl2  oxidizes  rapidly  in  the  air.    The  HC1  is  added  to  prevent  the 
precipitation  of  SbOCl. 

3.  Metallic  antimony  is  used,  rather  than  zinc  or  iron,  to  reduce  the 
tin  from  the  stannic  to  the  stannous  condition,  since  antimony  does 
not  reduce  the  tin  to  the  metallic  state  and  since  it  is  only  slowly  dis- 
solved by  boiling  HC1. 

4.  Just  as  metals  with  respect  to  conversion  into  their  ions  can  be 
arranged  in  a  series  hi  the  order  of  their  reducing  power  and  assigned 
certain  values,  the  specific  reduction-potentials,  expressing  this  power 
quantitatively,  so  reducing  substances  in  solution,  like  stannous  or 
ferrous  salts,  can  with  respect  to  their  conversion  into  a  higher  state  of 
oxidation,  thus  into  stannic  or  ferric  salts,  be  included  in  the  same  series 
with  appropriate  values  of  the  specific  reduction-potentials.     Such 
specific  reduction-potentials  represent  the  actual  reduction-potentials 
when  the  concentrations  of  both  the  ions  involved  (thus  of  Sn++++ 
and  Sn++,  or  of  Fe+++  and  Fe++)  are  equal.    For  each  ten-fold  decrease 
hi  the  ratio  of  the  concentration  of  the  ion  of  higher  valence  to  that  of 
the  ion  of  lower  valence,  the  actual  potential  increases  by  0.06  volt, 
if  the  difference  hi  the  two  valences  is  unity  (as  in  the  case  of  Fe++, 
Fe+++),  or  by  0.03  volt  if  the  valence  difference  is  two  (as  in  the  case 
of  Sn++,  Sn++++). 


P.  50  DETECTION  OF  PHOSPHATE  89 

DETECTION   OF  PHOSPHATE 

Procedure  50.  —  Detection  of  Phosphate.  —  Boil  5  cc.  of  the 
filtrate  from  the  H2S  precipitate  (P.  21)  till  the  H2S  is  expelled, 
pour  it  into  a  mixture  of  5  cc.  of  HN03  and  5  cc.  of  (NEU^MoOi 
reagent,  heat  the  mixture  to  60-70°,  and  let  it  stand  for  5-10 
minutes.  (Fine  yellow  precipitate,  presence  of  PHOSPHATE.) 
Reject  the  mixture. 

Notes.  —  i.  Phosphate  is  tested  for  here,  since  its  presence  may 
cause  the  precipitation  of  the  alkaline-earth  elements  (as  phosphates) 
with  the  aluminum-  and  iron-groups  (see  Note  6,  P.  51)  and  thus  make 
it  necessary  to  modify  somewhat  the  method  of  analysis  of  the  iron- 
group.  The  test  is  made  after  the  H2S  precipitation,  since  arsenate 
might  yield  a  similar  precipitate.  Incidentally  the  test  makes  it  un- 
necessary to  provide  for  the  detection  of  phosphate  in  the  process  for 
the  Detection  of  the  Acidic  Constituents. 

2.  The  precipitate  produced  is  the  triammonium  salt  of  a  com- 
plex phosphomolybdic  acid,  H+3PO4(MoOs)i2S.     Large  quantities  of 
(NH4)2Mo04  and  HN03  are  added,  so  as  to  convert  the  PO4S  ion  to  a 
large  extent  into  the  complex  anion.     A  large  quantity  of  NH4NOi 
is  also  added,  so  as  to  diminish  the  solubility  of  the  ammonium  phos- 
phomolybdate,  which  it  does  in  virtue  of  the  common-ion  effect.    The 
NEUNOs  is  added  with  the  reagent  itself,  which  is  a  solution  i  normal 
in  (NH^MoCX  and  3  normal  in  NH^NOs.    The  mixture  is  warmed,  so 
as  to  increase  the  rate  of  formation  of  the  complex  anion.    It  is  not 
boiled,  since  too  high  a  temperature  may  cause  the  precipitation  of 
white  MoO3  from  the  acidified  reagent. 

3.  Under  these  conditions   *"&   mg.   of  phosphate   (PO<)   can  be 
detected  in  the  5  cc.  of  solution  tested,  corresponding  to  ^  mg.  of  it  in 
the  whole  solution. 


PRECIPITATION  OF  ALUMINUM  AND  IRON  GROUPS    P.  51 


PRECIPITATION   AND   SEPARATION   OF   THE  ALUMINUM  AND 
IRON   GROUPS. 


TABLE  VII.  —  PRECIPITATION  AND  SEPARATION  OF  THE  ALUMINUM  AND 
IRON  GROUPS. 

Filtrate  from  the  Hydrogen  Sulfide  Precipitate. 
Add  NHtOH  in  excess  (P.  51). 

Precipitate* :  A1(OH)3,  Cr(OH)3,  Fe(OH)2-3 ;  Mn(OH)3  after  exposure  to  air. 
Solution :  Salts  of  Zn(NH3)4,  Ni(NH3)4,  Co,  Mn,  Ba,  Sr,  Ca,  Mg,  K,  and  Na. 
Add  (NH<)2S  and  filter  (P.  51}. 


Precipitate*  :  A1(OH),,  Cr(OH)3)  FeS,  ZnS,  MnS,  CoS,  NiS. 
Dissolve  in  HCl  and  KC103)  add  NaOH  (P.  52). 

Filtrate  : 
ALKALINE- 
EARTH  and 
ALKALI 
GROUPS. 

Precipitate*:  Fe(OH)3,  Mn(OH)2,  Co(OH)2,  Ni(OH)2. 
Solution  :  NaA102)  NaCrO2,  Na2Zn02.     " 
Add  Nch02  and  filter  (P.  52). 

Filtrate  :  ALUMINUM-  GROUP. 
NaA102,  Na2Zn02)  Na2CrO4. 
See  Table  VIII. 

Precipitate*:  IRON-GROUP. 
MnO(OH)2,  Fe(OH)3, 
Co(OH)»,  Ni(OH)«. 
See  Table  EX. 

*  When  phosphate  is  present  in  the  solution,  these  precipitates  may  contain  the  phos- 
phates of  the  elements  otherwise  precipitated  as  hydroxides,  and  also  the  phosphates  of 
barium,  strontium,  calcium,  and  magnesium. 

Procedure  51.  —  Precipitation  of  the  Aluminum  and  Iron 
Groups.  —  Boil  the  remainder  of  the  filtrate  from  the  H2S  pre- 
cipitate (P.  21)  till  the  H2S  is  expelled,  make  it  alkaline  with 
NH4OH  (see  Note  i,  P.  34),  and  heat  the  mixture  to  boiling. 
(No  precipitate,  absence  of  ALUMINUM,  CHROMIUM,  IRON,  and 

ALKALINE-EARTH  PHOSPHATE.)      Add  6  n.  (NH4)2S  solution,  I  CC. 

at  a  time  (or  if  nickel  seems  to  be  present,  pass  in  H^S  a  minute 
at  a  time),  till  after  shaking  the  flask  the  vapors  slightly  darken 
paper  moistened  with  PbAc2  solution.  Heat  the  mixture  nearly 
to  boiling,  shake  it,  and  let  it  stand  2  or  3  minutes.  (Precipitate, 
presence  of  ALUMINUM-GROUP  or  IRON-GROUP  or  of  ALKALINE- 
EARTH  PHOSPHATE.)  Filter,  using  suction  if  the  precipitate  is 


P. 51   PRECIPITATION  OF  ALUMINUM  AND  IRON  GROUPS    91 

large,  and  wash  the  precipitate,  first  with  water  containing  about 
i%  of  the  (NH^S  solution,  and  then  with  a  little  pure  water. 
If  the  nitration  is  slow,  keep  the  funnel  covered  with  a  watch- 
glass.  To  the  filtrate  add  a  few  drops  of  (NH4)2S  solution, 
and  boil  the  mixture  for  a  few  seconds  (or,  in  case  it  is  dark 
colored,  until  it  becomes  colorless  or  light  yellow) ;  filter  out 
any  precipitate,  uniting  it  with  the  preceding  one.  (Precipitate, 
P.  52  ;  filtrate,  P.  71.) 

Notes.  —  i.  The  purposes  of  the  various  operations  are  as  follows: 
The  H2S  is  boiled  out  so  that  the  effect  of  the  addition  of  NH4OH  alone 
may  be  noted ;  for  it  often  gives  useful  indications  as  to  what  elements 
are  present  or  absent.  Only  a  slight  excess  of  (NH4)2S  is  used,  in 
order  to  prevent  so  far  as  possible  dissolving  NiS.  By  passing  H2S 
into  the  ammoniacal  solution,  instead  of  adding  (NH4)2S,  the  dissolving 
of  NiS  may  be  almost  entirely  prevented ;  therefore,  though  the 
operation  takes  a  little  longer,  the  use  of  H2S  is  to  be  preferred  when 
nickel  seems  to  be  present.  The  mixture  is  shaken  in  order  to  coagulate 
the  precipitate  and  make  it  filter  more  readily.  The  heating  also 
promotes  the  coagulation  of  the  precipitate.  To  the  filtrate  a  few 
drops  of  the  (NH4)2S  solution  are  added  to  make  sure  an  excess  is 
present.  The  filtrate  is  boiled  for  a  few  moments  to  insure  the  com- 
plete precipitation  of  Cr(OH)3,  or  longer  to  insure  that  of  NiS,  whose 
presence  is  indicated  by  a  brown  or  nearly  black  color  of  the  filtrate. 
Finally,  it  is  directed  to  wash  with  water  containing  a  little  (NH4)2S 
and  to  keep  the  filter  covered,  in  order  that  some  excess  of  (NH4)2S 
may  always  be  present ;  for,  if  the  (NH4)2S  adhering  to  the  precipitate 
is  removed  by  oxidation  or  by  volatilization  (as  H2S  and  NH3),  the 
sulfides  are  oxidized  to  soluble  sulfates  by  the  air.  If  suction  is  used, 
for  the  same  reason  care  must  be  taken  not  to  suck  air  through  the 
precipitate. 

2.  By  a  moderate  excess  of  NH4OH  in  the  presence  of  ammonium 
salt  the  trivalent  elements,  aluminum,  chromium,  and  ferric  iron,  are 
completely  precipitated ;  while  the  bivalent  elements,  zinc,  manganese, 
nickel,  cobalt,  ferrous  iron,  and  the  alkaline-earth  elements,  remain 
hi  solution  (except  that  when  a  large  quantity  of  cobalt  is  present,  it 
may  be  partially  precipitated  as  a  basic  salt).  Ferrous  and  manganous 
salts  are,  however,  rapidly  oxidized  hi  alkaline  solution  by  the  oxygen 
of  the  air ;  and  the  higher  hydroxides  are  then  precipitated.  Cobaltous 
salts  in  ammoniacal  solution  are  also  oxidized  by  the  ah-;  but  the 
cobaltic  hydroxide  remains  in  solution.  When  chromium  is  present, 


92    PRECIPITATION  OF  ALUMINUM  AND  IRON  GROUPS    P.  51 

zinc  and  magnesium  may  be  completely  precipitated  in  combination 
with  it  as  chromites  (ZnCrO2  and  MgCr02).  If  a  large  excess  of 
NH4OH  is  used,  a  few  milligrams  of  aluminum  and  chromium  may  be 
dissolved,  the  latter  in  larger  quantity  if  the  mixture  be  not  heated  to 
boiling. 

3.  In  view  of  these  facts,  the  conclusion  may  be  drawn  that  alu- 
minum, chromium,  and  iron  are  absent  in  case  the  NH4OH  does  not 
produce  a  precipitate  that  is  perceptible  even  after  heating  the  mixture 
to  boiling,  shaking  it,  and  allowing  it  to  stand.     These  operations 
insure  the  precipitation  of  chromium,  and  at  least  the  partial  oxida- 
tion of  iron  to  the  ferric  state  and  its  precipitation.    The  heating  and 
shaking  serve  also  to  cause  the  precipitate  to  coagulate  hi  flocks,  thereby 
making  perceptible  a  small  precipitate,  which  might  otherwise  escape 
detection  on  account  of  its  transparency. 

4.  The  hydroxide  of  aluminum  is  white ;  that  of  chromium,  grayish- 
green.    The  color  of  the  precipitated  iron  hydroxide  varies  with  the 
state  of  oxidation  of  the  iron;   pure  ferrous  salts  yielding  (if  treated 
with  NH4OH  in  the  absence  of  ammonium  salts)  a  white  precipitate, 
and  ferric  salts  a  dark  red  one,  while  mixtures  of  ferrous  and  ferric  salts 
give  green  or  black  precipitates.    In  the  alkaline  mixture  the  precipitate 
produced  with  ferrous  salts  gradually  undergoes  corresponding  changes 
in  color  as  a  result  of  progressive  oxidation.    The  precipitate,  consist- 
ing of  Mn(OH)3  and  MnO(OH)2,  which  manganese  produces  as  it 
becomes  oxidized  has  a  dark  brown  color.    With  the  excess  of  NH^OH 
nickel  gives  a  blue  solution,  cobalt  a  purplish-red  solution,  and  chro- 
mium (in  case  it  dissolves)  a  pink  solution.     Upon  oxidation  the  cobalt 
solution  becomes  dark  brown.    The  precipitate  produced  with  a  large 
quantity  of  cobalt  is  blue,  changing  to  a  bright  green  on  oxidation. 

5.  The  influence  of  an  excess  of  the  NH4OH  and  of  the  presence  of 
ammonium  salt  on  the  solubilities  of  the  various  hydroxides  is  ex- 
plained by  the  mass-action  law  and  ionic  theory  as  follows :   In  order 
that  any  hydroxide,  say  of  the  type  M(OH)2,  may  be  precipitated,  it 
is  necessary  that  the  product  (M++)X(OH~)2  of  the  concentrations  of 
the  ions  M++  and  OH~  in  the  solution  under  consideration  attain  the 
saturation-value  of  that  product.    This  saturation-value  varies,  of 
course,  with  the  nature  of  the  hydroxide ;  but  for  all  the  elements  of 
the  aluminum-  and  iron-groups  and  for  magnesium,  it  is  so  small  that 
hi  a  solution  containing  in  100  cc.  only  i  or  2  mg.  of  the  element  and  a 
slight  excess  of  NH4OH,  the  product  (M++)X(OH-)2  exceeds  it,  and 
precipitation  results.    When,  however,  much  ammonium  salt  is  also 
present,  this  greatly  reduces,  in  virtue  of  the  common-ion  effect,  the 
ionization  of  the  NH4OH  and  therefore  the  OH~  concentration  in  the 
solution,  so  that  now  for  certain  elements  the  product  (M++)X(OH~)2 


P.  51    PRECIPITATION  OF  ALUMINUM  AND  IRON  GROUPS    93 

does  not  reach  the  saturation- value,  even  when  (M++)  is  moderately 
large  (say  500  mg.  in  100  cc.).  This  is  true  of  ferrous  iron,  manganese, 
cobalt,  and  magnesium;  but  in  the  cases  of  the  trivalent  elements, 
aluminum,  chromium,  and  ferric  iron,  the  solubility  of  the  hydroxides 
in  water  is  so  slight  that  even  in  ammonium  salt  solutions  the  solu- 
bility is  not  appreciable. 

|"  If  these  were  the  only  considerations  involved,  the  greater  the  ex- 
cess of  NH4OH  added,  the  less  would  be  the  solubility  of  any  hy- 
droxide; but  other  influences  of  two  kinds  come  into  play  with  cer- 
tain of  the  elements. 

The  first  of  these  influences  is  shown  by  zinc  and  nickel.  In  the 
case  of  these  elements,  just  as  with  cadmium  and  copper,  the  excess  of 
ammonia  combines  with  the  simple  cation  M++,  forming  complex 
cations  of  the  type  M(NH3)4++,  thereby  removing  the  simple  cation 
from  the  solution  and  making  it  necessary  for  more  of  the  hydroxide 
to  dissolve  in  order  to  bring  back  the  value  of  (M++)X(OH~)2  to  the 
saturation-value.  In  such  a  case  the  presence  of  ammonium  salt 
increases  the  solubility  still  further,  since  it  greatly  decreases  the 
value  of  (OH~),  owing  to  the  common-ion  effect  on  the  ionization  of 
the  NH4OH.  Chromium  also  forms  similar  ammonia  complexes,  but 
hi  much  smaller  proportion. 

The  second  kind  of  influence  is  exhibited  in  the  case  of  AlOjHj. 
This  hydroxide  is,  like  SnO2H2,  an  example  of  an  amphoteric  substance ; 
for  it  behaves  both  as  a  base  and  as  an  acid  in  consequence  of  its  being 
appreciably  ionized  both  into  OH~  and  A1+++  and  into  H+  and  A1O8H2- 
(or  into  H+,  AlOr,  and  H20).  With  the  H+  arising  from  the  latter 
form  of  ionization  .the  OH~  coming  from  the  excess  of  NH4OH  com- 
bines to  form  H20,  so  as  to  satisfy  the  mass-action  expression  for  the 
ionization  of  water,  (H+)  X  (OH~)  =  a  constant  (which  has  the  very  small 
value  icr14  at  25°).  This  causes  more  A10SH8  to  dissolve  until  the 
product  (AlOr)  X  (H+)  again  attains  its  saturation-value.  This  shows 
that  the  quantity  of  aluminum  dissolved  increases  with  the  OH~  con- 
centration in  the  solution,  and  that  therefore  it  would  be  much  greater 
in  a  solution  of  a  largely  ionized  base  like  NaOH  than  in  that  of  a  slightly 
ionized  base  like  NH4OH.  It  also  shows  that  the  presence  of  am- 
monium salts  tends  to  neutralize  the  solvent  action  of  an  excess  of 
NH4OH,  since  they  decrease  the  OH~  concentration  in  its  solution. 

6.  When  phosphate  is  present,  magnesium,  calcium,  strontium, 
barium,  and  manganese  may  be  partially,  or  even  completely,  pre- 
cipitated by  NH4OH.  The  reasons  for  this  are  as  follows.  The 
normal  phosphates  and  the  mono-hydrogen  phosphates  of  these  ele- 
ments are  only  slightly  soluble  in  water,  but  dissolve  readily  in  acids, 
owing  to  the  conversion  of  the  P0«=  and  HPO4=  ions  into  H2P04~  and 


94   PRECIPITATION  OF  ALUMINUM  AND  IRON  GROUPS    P. 51 

(unionized)  H3P04  by  the  H+  ion  of  the  added  acid.  Upon  the  addition 
of  an  excess  of  NH4OH  to  such  a  solution  these  acid  compounds  are  re- 
converted into  the  normal  phosphates,  and  these  are  reprecipitated. 
It  is  therefore  necessary,  when  phosphate  is  present,  to  provide  for 
the  detection  of  the  alkaline-earth  elements  in  the  analysis  of  the 
NH4OH  precipitate.  They  are,  however,  not  necessarily  found  in 
that  precipitate;  for,  when  other  elements,  like  iron  and  aluminum, 
which  form  much  less  soluble  phosphates,  are  also  present,  these  may 
combine  with  the  phosphate-ion  present,  thus  leaving  the  alkaline- 
earth  elements  in  solution. 

7.  There  is  one  other  inorganic  acidic  constituent,  namely  fluoride, 
which,  like  phosphate,  may  cause  the  alkaline-earth  elements  to  pass 
into  the  NH4OH  precipitate;  for  barium,  strontium,  and  magnesium 
fluorides  are  fairly  soluble  and  calcium  fluoride  is  slightly  soluble  in 
dilute  HN03  without  expulsion  of  the  HF  (unless  the  solution  is  evapo- 
rated), and  they  are  much  less  soluble  in  dilute  NH4OH  solution.    It 
is,  however,  not  important  to  make  special  provision  for  the  detection 
of  alkaline-earth  elements  in  the  (NH4)2S  precipitate  in  the  presence 
of  fluoride,  since  enough  barium,  strontium,  and  magnesium  remain  in 
solution  to  be  detected  by  the  usual  procedure,  and  since  substances 
containing  the  slightly  soluble  CaF2  are  likely  to  require  evaporation 
with  concentrated  acids  in  the  Preparation  of  the  Solution  (in  P.  3 
or  5),  whereby  the  fluoride  is  expelled.    Borate  also  forms  with  the 
salts  of  alkaline-earth  elements  salts  which  may  require  acid  for  their 
solution ;  but  these  salts  are  soluble  enough  so  that  they  are  not  pre- 
cipitated from  the  large  volume  of  hot  solution  containing  ammonium 
salts  to  which  NH4OH  is  added  in  P.  51.     Many  organic  acidic  con- 
stituents would  cause  precipitation  of  the  alkaline-earth  elements; 
but  these  will  have  been  detected  in  the  closed-tube  test  (in  P.  i)  and 
will  have  been  destroyed  in  the  Preparation  of  the  Solution  by  the 
treatment  of  the  substance  with  HN03  and  H,SO4  (in  P.  8).    Oxalate 
is,  however,  a  common  organic  constituent  whose  salts  do  not  char 
much  on  heating,  and  one  whose  presence  may  cause  complete  pre- 
cipitation of  calcium  and  partial  precipitation  of  barium  and  strontium 
upon  addition  of  NH4OH  to  the  acid  solution.    Provision  is  therefore 
made  (in  P.  65)  for  the  detection  of  alkaline-earth  elements,  when  the 
substance  is  of  such  a  character  that  it  may  contain  oxalate. 

8.  (NH4)2S  precipitates  ZnS,  MnS,  NiS,  CoS,  and  FeS,  and  converts 
Fe(OH)s  into  Fe2S3.    The  hydroxides  of  aluminum  and  chromium  are 
not  affected  by  the  (NH4)2S. 

9.  The  sulfides  of  iron,  nickel,  and  cobalt  are  black ;  ZnS  is  white ; 
and  MnS  is  flesh-colored,  but  turns  brown  on  standing  in  the  air,  owing 
to  oxidation  to  Mn(OH)3  and  MnO(OH)2. 


P.  62      SEPARATION  OF  ALUMINUM  AND  IRON  GROUPS      95 

10.  When  nickel  is  present  alone  or  when  it  forms  a  large  proportion 
of  the  (NH4)2S  precipitate,  several  milligrams  of  it  usually  pass  into 
the  filtrate,  giving  it  a  brown  or  black  color ;  and  some  NiS  also  passes 
through  the  filter  with  the  wash-water.  In  this  case  it  is  useless  to 
try  to  remove  the  NiS  by  filtering  again,  but  it  can  be  coagulated  by 
boiling  for  several  minutes.  The  brown  solution  is  formed  only  in  the 
presence  of  ammonium  disulfide,  (NH4)2S2.  Its  formation  can,  as 
stated  above,  be  avoided  by  passing  HZS  into  the  NH4OH  solution, 
instead  of  adding  the  (NH4)2S  reagent,  which  after  exposure  to  the  air 
always  contains  some  disulfide.  The  nature  of  the  brown  solution  is 
not  known. 

'Procedure  52.  —  Separation  of  the  Aluminum-Group  from  the 
Iron-Group.  —  Transfer  the  (NHOaS  precipitate  (P.  51)  to  a 
casserole  (see  Note  i,  P.  22),  add  5-15  cc.  of  HC1,  stir  the  mix- 
ture for  a  minute  or  two  in  the  cold,  and  then  boil  it  for  1-2 
minutes.  If  there  is  a  black  residue,  sprinkle  into  the  hot,  but 
not  boiling,  solution  0.1-0.3  cc-  of  powdered  KC1O3,  and  heat 
it  nearly  to  boiling  for  1-2  minutes.  Add  5-10  cc.  of  water, 
filter  out  the  sulfur  residue,  and  evaporate  the  nitrate  almost  to 
dryness. 

Dilute  the  solution  to  10-20  cc.,  and  make  it  alkaline  with 
NaOH  solution  (see  Note  i,  P.  23),  adding  10-20  cc.  more  water 
if  the  mixture  becomes  thick  with  the  precipitate.  Place  the 
casserole  in  a  vessel  of  cold  water,  and  gradually  sprinkle  into 
the  mixture  from  a  dry  lo-cc.  graduate  1-3  cc.  of  Na2O2  powder, 
with  constant  stirring.  (In  case  phosphate  was  found  present 
(in  P.  50),  add  also  5  cc.  of  3  n.  Na2CO3  solution.)  Boil  the 
mixture  for  2-3  minutes,  cool  it,  and  dilute  it  to  about  60  cc. 
(Precipitate,  presence  of  IRON-GROUP.)  Filter  through  a  hardened 
filter,  using  suction;  wash  the  precipitate  thoroughly  with  hot 
water,  and  suck  it  as  dry  as  possible.  (Precipitate,  P.  61 ; 
nitrate,  P.  53.) 

Notes.  — i.  All  the  hydroxides  and  all  the  sulfides,  except  NiS 
and  CoS,  when  freshly  precipitated,  dissolve  readily  in  cold  HC1.  If, 
therefore,  there  is  considerable  black  residue  after  adding  the  HC1, 
it  shows  the  presence  of  nickel  or  cobalt ;  a  very  small  black  residue 
may,  however,  be  due  to  FeS  enclosed  within  sulfur.  The  fact  that 
there  is  no  such  dark-colored  residue  does  not,  however,  prove  that 


96       SEPARATION  OF  ALUMINUM  AND  IRON  GROUPS    P.  52 

nickel  and  cobalt  are  entirely  absent ;  for  a  considerable  quantity  of 
them  (even  5  mg.)  may  dissolve  completely  in  the  HC1  when  large 
quantities  of  other  elements,  especially  iron,  are  also  present. 

2.  The  (NH4)2S  precipitate  is  first  treated  with  HC1,  partly  in  order 
to  furnish  the  indication  just  referred  to  of  the  presence  of  nickel  or 
cobalt,  but  also  because  much  more  free  sulfur  and  sulfate  would  be 
formed  by  oxidation  if  KC1O3  (or  HN03)  were  used  with  the  HC1  at 
the  start.     (The  formation  of  sulfate  would  cause  the  precipitation 
of  barium  if  that  element  was  precipitated  by  NH4OH  because  of  the 
presence  of  phosphate.)    If  NiS  or  CoS  is  present  in  the  residue, 
KC103  must,  however,  be  subsequently  added,  to  insure  the  solution 
of  these  sulfides. 

3.  By  NaOH,  iron,  manganese,  nickel,  and  cobalt  are  completely 
precipitated  and  do  not  dissolve  in  moderate  excess ;  while  aluminum, 
chromium,  and  zinc  remain  in  solution  or  dissolve  when  a  sufficient 
excess  is  added.    The  solubility  of  the  last  three  elements  is  due  to 
the  fact  that  their  hydroxides  are  amphoteric  substances  which  form 
with  the  NaOH  soluble  aluminate   (NaA102),   chromite   (NaCr02), 
and  zincate  (Na2Zn02),  respectively.    When  zinc  and  chromium  are 
simultaneously  present  they  are  precipitated  in  the  form  of  zinc  chro- 
mite, Zn(Cr02)2.     Chromium  would  also  be  completely  precipitated, 
owing  to  hydrolysis  of  the  chromite  and  the  formation  of  a  less  soluble 
solid  hydroxide,  if  the  NaOH  solution  were  boiled  before  adding  Na2O2. 
Mn(OH)2  is  white,  but  rapidly  turns  brown,  owing  to  oxidation  to 
Mn(OH)3 ;   Ni(OH)2  is  light  green ;    Co(OH)2  is  pink,  but  from  cold 
cobaltous  salt  solutions  a  blue  basic  salt  is  first  precipitated.    If  a 
large  excess  of  NaOH  be  added,  a  little  Co(OH)2  dissolves,  yielding  a 
blue  solution,  doubtless  forming  a  salt  such  as  Na2Co02.    This  is  to 
be  avoided,  since  then  the  cobalt  will  not  be  completely  oxidized  and 
precipitated  upon  the  subsequent  addition  of  Na2O2. 

4.  By  the  addition  of  Na202,  Fe(OH)2  is  changed  to  dark  red  Fe(OH)3, 
Mn(OH)2  to  brown  hydrated  Mn02,  and  Co(OH)2  to  black  Co(OH)3, 
all  of  which  are  insoluble  in  excess  of  NaOH.     Chromium,  which  after 
the  addition  of  cold  NaOH  is  present  as  soluble  sodium  chromite 
(NaCrO2),  is  converted  by  Na2O2  into  chromate  (Na2CrO4).    This  re- 
mains in  solution  together  with  the  zinc,  which  is  still  present  as  zincate. 

5.  Even  a  cold  solution  of  Na202  decomposes  slowly  with  evolution 
of  oxygen,  and  this  decomposition  takes  place  with  explosive  violence 
when  the  solution  is  hot.    The  peroxide  is  therefore  added  in  small 
portions  to  the  cold  solution.    It  is  best  to  transfer  a  little  of  it  from 
the  can  in  which  it  comes  in  trade  directly  (without  using  paper)  to  a 
dry  lo-cc.  graduate,  and  then  to  sprinkle  it  into  the  solution  with 


P.  52     SEPARATION  OF  ALUMINUM  AND  IRON  GROUPS       97 

constant  stirring.  A  steady  evolution  of  gas  after  the  mixture  has  been 
well  stirred  is  an  indication  that  sufficient  peroxide  has  been  added. 
When  much  chromium  is  present,  it  should  be  added  till  the  green 
precipitate  disappears  and  the  liquid  assumes  a  dark  yellow  color. 
The  solution  is  diluted  before  filtering  in  order  to  avoid  the  disinte- 
gration of  the  filter-paper.  A  hardened  filter  is  used,  because  it  is  less 
attacked  by  strongly  alkaline  solutions;  also  because  an  ordinary 
filter  would  disintegrate  when  treated  in  P.  61  with  16  n.  HN03,  and 
by  its  reducing  action  might  prevent  the  precipitation  of  a  small 
quantity  of  manganese. 

6.  This  separation  with  NaOH,  Na2O2,  and  Na2CO»  is  a  very 
satisfactory  one,  except  in  the  case  of  zinc.    As  much  as  5  mg.  of  this 
element  is  almost  completely  carried  down  in  the  precipitate  when 
much  iron,  nickel,  or  cobalt  is  present ;   and  as  much  as  20  mg.  of  it 
may  be  completely  precipitated  when  much  manganese  is  present. 
Provision  is  therefore  made  (in  P.  65-66)  for  the  detection  of  zinc  in 
the  precipitate. 

7.  The  Na2C03  is  added  to  insure   the  complete  precipitation  of 
magnesium,  calcium,  strontium,  and  barium,  whose  hydroxides,  es- 
pecially that  of  barium,  are  somewhat  soluble  even  in  the  presence  of 
NaOH.    ZnCO3,  though  insoluble  in  a  dilute  solution  of  Na2CO3  alone, 
dissolves  when  much  NaOH  is  present,  owing  to  nearly  complete  con- 
version of  the  zinc-ion  into  zincate-ion  (Zn02=).    The  Na2CO3  also 
serves  to  metathesize  barium  and  strontium  chromates,  which  would 
otherwise  carry  chromium  into  the  precipitate  and  prevent  its  de- 
tection (in  P.  57).    The  Na2COs  solution  is  added  only  when  phos- 
phate has  been  detected  in  P.  50,  since  its  addition  is  superfluous  when 
alkaline-earth  elements  cannot  be  present  in  the  NaOH  solution. 

8.  Phosphate,  if  present,  divides  itself  in  this  procedure  between 
the  precipitate  and  solution  in  a  proportion  which  depends  on  the 
nature  and  quantities  of  the  basic  elements  present.     (See  Note  6, 
P.  51.)     Its  presence  does  not  cause  any  of  the  elements  to  precipitate 
which  would  not  otherwise  do  so,  in  spite  of  the  slight  solubility  of 
aluminum  and  zinc  phosphates.    This  is  due  to  the  fact  that  the 
cations  of  these  elements  (Al+++,  Zn++)  are  present  in  the  NaOH  solu- 
tion only  at  an  extremely  small  concentration,  owing  to  their  con- 
version by  the  OH~  into  anions  (A102~,  Zn02=). 


98 


ANALYSIS  OF  ALUMINUM-GROUP 


P.5S 


ANALYSIS   OF   THE  ALUMINUM-GROUP 


TABLE  VIII.  —  ANALYSIS  OF  THE  ALUMINUM-GROUP. 


Filtrate  from  the  Sodium  Hydroxide  and  Peroxide  Treatment : 

,  Na2ZnO2,  Na2CrO4.    Acidify  with  HCl,  add  NHtOH  (P.  53). 


Precipitate:  A1(OH)3. 
Dissolve  in  HN03, 
add  Co(N03)2,  evapo- 
rate, ignite  (P.  54}. 

Filtrate:  Zn(NH3)4Cl2,  Na2CrO4. 
Add  NatCOz,  boil  to  expel  NH3  (P.  55). 

Precipitate  : 
ZnCO3*Zn(OH)2. 
Dissolve  in  HCl,  add 
NH4OH  and  (NHt)*S 
(P>  5<5)- 

Filtrate:  Na2CrO4. 
Add  HAc  and  PbAct 
(P-  57). 

Blue  residue  : 
Co(A102)2. 

Yellow  precipitate  : 
PbCrO4. 

White  precipitate  :  ZnS. 

Procedure  53.  —  Separation  of  Aluminum  from  Chromium 
and  Zinc.  —  Acidify  the  alkaline  solution  (P.  52)  by  adding 
HCl,  5  cc.  at  first  and  then  2  cc.  at  a  time,  cooling  after  each 
addition.  Add  5  cc.  of  3  n.  NH4C1  solution,  and  NH4OH  till 
the  mixture  after  shaking  smells  of  it,  then  5  cc.  more;  and 
heat  almost  to  boiling.  (White  flocculent  precipitate,  presence 
of  ALUMINUM.)  Filter,  and  wash  the  precipitate.  (Precipitate, 
P.  54;  nitrate,  P.  55.) 

Notes.  —  i.  In  acidifying  the  solution  with  HCl  care  must  be  taken 
to  keep  the  solution  cool  and  to  avoid  adding  much  excess ;  since  other- 
wise chromium  may  be  reduced  from  the  state  of  chromate  to  that  of 
chromic  salt.  HNOj  is  not  used  for  the  acidification,  since  it  commonly 
contains  HNO2,  which  would  instantly  reduce  the  chromate. 

2.  If  the  chromate  is  reduced  by  the  HCl,  or  if  the  chromic  salt  was 
incompletely  oxidized  to  chromate  in  the  Nai02  treatment  (in  P.  52), 
green  Cr(OH)3  will  be  precipitated  by  NH4OH,  together  with  the 
Al(OH),. 

3.  NH4C1  and  a  moderate  excess  of  NH4OH  are  added  so  as  to  keep 
the  zinc  in  solution,  through  the  formation  of  the  complex  Zn(NH3)4++ 
ion,  in  accordance  with  the  principles  described  in  Note  5,  P.  51.    A 
large  excess  of  NH4OH  is  avoided,  since  it  would  dissolve  a  little 
Al(OH),,  owing  to  formation  of  NH^O,. 


P.  54  ANALYSIS  OF  ALUMINUM-GROUP  99 

4.  A  definite  excess  of  HC1  (of  not  more  than  2  cc.)  is  used  in  neu- 
tralizing the  alkaline  solution,  and  a  measured  volume  of  NH4C1  solu- 
tion is  added,  so  that  a  proper  quantity  of  NajCOj  solution  may  be 
used  in  precipitating  zinc  in  P.  55. 

5.  Since  aluminum  and  silica  are  very  likely  to  be  present  in  the 
NaOH  and  NajOj  used  as  reagents,  a  blank  test  for  these  impurities 
should  be  made  whenever  new  reagents  are  employed  for  the  first 
time,  by  treating  10  cc.  of  the  NaOH  reagent,  or  2  g.  of  Na2O2  added 
to  20  cc.  of  water,  by  P.  53,  and  comparing  the  NH4OH  precipitate 
with  that  obtained  in  the  actual  analysis.    It  is  also  well  at  the  same 
tune  to  test  for  zinc  by  passing  H2S  into  the  ammoniacal  solution.    If 
the  NaOH  available  is  found  to  contain  aluminum  or  silica,  an  addi- 
tional quantity  (0.5  cc.  more)  of  NajOj  may  be  used  in  place  of  it  in 
P.  52. 

Procedure  54.  —  Confirmatory  Test  for  Aluminum.  —  Dis- 
solve the  NH4OH  precipitate  (P.  53),  or  a  small  quantity  of  it 
if  it  is  large,  in  5  cc.  of  2  n.  HNOs.  To  the  solution  add  5-10  cc. 
of  water,  2-15  drops  of  0.3  n.  Co(NOa)2  solution,  and  3  cc.  of 
NBUOH ;  and  heat  the  mixture  nearly  to  boiling.  Filter  with 
the  aid  of  suction,  and  wash  the  precipitate  with  water,  finally 
sucking  it  as  dry  as  possible.  Open  the  filter  paper,  tear  off  the 
portions  to  which  no  precipitate  adheres,  make  a  small  roll  of 
the  remainder,  wind  a  platinum  wire  around  it  in  the  form  of  a 
spiral,  heat  the  paper  in  a  flame  till  the  carbon  is  burnt  off,  and 
ignite  it  for  a  minute  or  two  at  a  bright  red  heat.  (Blue  residue, 
presence  of  ALUMINUM.) 

Notes.  —  i.  A  confirmatory  test  for  aluminum  should  always  be 
tried  when  the  NH4OH  precipitate  is  small ;  for  the  precipitation  by 
NH<OH  of  an  element  whose  hydroxide  is  soluble  in  NaOH  is  not  very 
characteristic  (lead,  antimony,  tin,  and  silicon  showing  a  similar  be- 
havior). It  is  especially  necessary  to  guard  against  mistaking  SiO»Hi 
for  A1(OH)3;  for  the  former  substance,  if  not  entirely  removed  in  the 
process  of  preparing  the  solution,  may  appear  at  this  point. 

2.  The  confirmatory  test  with  Co(NO3)2  depends  upon  the  formation 
of  a  blue  substance,  whose  formula  is  not  definitely  known ;  but  it  is 
doubtless  a  compound  of  the  two  oxides  CoO  and  A12O3,  and  is  probably 
cobalt  aluminate,  Co(A102)2.  It  enables  0.5  mg.  Al  to  be  detected,  or 
even  0.2  mg.  after  a  little  practice.  No  other  element  gives  a  blue 
color  to  the  ash.  It  is  essential  to  have  the  aluminum  present  in 
excess ;  for  otherwise  the  blue  color  is  obscured  by  the  black  oxide  of 


100  ANALYSIS  OF  ALUMINUM-GROUP  P.  55 

cobalt.  Moreover,  when  sodium  salts  are  present,  the  ash  fuses  to- 
gether and  the  test  is  unsatisfactory.  For  this  reason  the  first  NH<OH 
precipitate  (P.  53)  is  not  only  washed,  but  it  is  dissolved,  the  alu- 
minum is  reprecipitated,  and  the  new  precipitate  is  washed,  to  insure 
the  complete  removal  of  the  sodium  salts.  The  ammonium  salts  that 
may  be  retained  by  the  second  precipitate  do  no  harm,  as  they  volatilize 
when  the  precipitate  is  ignited. 

3.  Although  soluble  in  the  ammoniacal  solution,  a  portion  of  the 
cobalt  added  is  carried  down  by  the  A1(OH)3  precipitate  by  being  de- 
posited on  the  surface  of  its  particles  —  a  phenomenon  known  as 
adsorption,  which  is  especially  pronounced  hi  the  case  of  gelatinous 
precipitates  in  contact  with  alkaline  liquids.  In  this  case,  it  serves 
in  some  measure  to  adjust  the  quantity  of  cobalt  associated  with  the 
aluminum  to  the  size  of  the  precipitate ;  but  care  should  also  be  taken 
to  add  a  number  of  drops  of  the  Co(NO3)2  solution  roughly  propor- 
tional to  the  amount  of  the  precipitate  subjected  to  the  test  —  using 
the  lower  limit,  2  drops,  when  it  contains  1-5  mg.  of  aluminum,  and 
the  upper  limit,  15  drops,  when  it  contains  50-100  mg.  of  aluminum. 

Procedure  55.  —  Detection  of  Zinc.  —  To  the  NHUOH  solu- 
tion (P.  53)  add  15  cc.  of  3  n.  Na2COs  solution,  and  evaporate 
in  a  casserole  (to  15-20  cc.)  till  the  solution  no  longer  smells  of 
ammonia.  (White  precipitate,  presence  of  ZINC.)  Wash  the 
precipitate.  (Precipitate,  P.  56 ;  nitrate,  P.  57.) 

Notes.  —  i.  The  precipitate  produced  by  Na2CO3  is  a  basic  zinc 
carbonate,  ZnCO3  •  #Zn(OH)2,  containing  carbonate  and  hydroxide  in 
proportions  that  vary  with  the  conditions  of  the  precipitation.  Since 
it  becomes  compact  during  the  boiling,  even  a  slight  precipitate  must 
not  be  disregarded.  Moreover,  since  there  is  usually  a  small  white 
precipitate  of  silica,  coming  from  the  reagents  or  from  action  of  the 
alkali  on  the  dishes,  the  confirmatory  test  described  in  P.  56  must  be 
made  if  any  precipitate  whatever  separates. 

2.  The  solution  is  evaporated  to  a  small  volume  in  order  to  decom- 
pose the  ammonium  salts  and  expel  the  NH3  completely;    for  even 
the  very  slightly  soluble  basic  ZnCO3  is  dissolved  by  NH4OH,  owing  to 
the  small  dissociation  of  the  Zn(NH3)<++  complex. 

3.  Enough  Na2CO3  solution  must  be  added  to  react  with  all  the 
ammonium  salt  present  and  to  furnish  about  5  cc.  additional  to  pre- 
cipitate the  zinc.    If  an  excess  of  2  cc.  of  HC1  was  used  in  P.  53  in 
neutralizing  the  alkaline  solution,  and  if  5  cc.  of  3  n.  NH4C1  solution 
were  added  as  directed,  9  cc.  of  the  Na2CO3  solution  will  be  needed  to 
destroy  the  ammonium  salt,  and  15  cc.  in  all  will  be  sufficient. 


P.  57  ANALYSIS  OF  ALUMINUM-GROUP  101 

Procedure  56.  —  Confirmatory  Test  for  Zinc.  —  Pour  through 
the  filter  containing  the  Na2C03  precipitate  (P.  55)  10  cc.  of 
3  n.  HC1.  Add  HN4OH  to  the  solution  till  its  odor  is  perceptible, 
and  then  3  cc.  more ;  and  heat  the  mixture  nearly  to  boiling. 
Filter  out  any  precipitate,  and  add  0.5-2.0  cc.  of  6  n.  (NH^S 
solution.  (White  precipitate,  presence  of  ZINC.)  If  the  result  is 
doubtful,  treat  the  precipitate,  or  a  small  portion  of  it  if  it  is 
large,  by  P.  67.  Reject  the  filtrate. 

Notes.  —  i.  The  excess  of  NH4OH  is  added  to  redissolve  Zn(OH)i, 
which  may  separate  when  a  large  quantity  of  zinc  is  present.  The 
solution  is  then  filtered,  unless  perfectly  clear,  to  remove  any  Al(OH)s, 
Cr(OH)3,  or  H2SiOs  which  may  still  be  present. 

2.  When  a  white  flocculent  precipitate  or  (with  a  small  quantity  of 
zinc)  a  translucent  turbidity  is  produced  by  the  (NH<)2S,  it  is  sufficient 
evidence  of  the  presence  of  zinc.  But,  if  a  darkened  precipitate  is 
produced,  the  presence  of  zinc  in  it  should  be  confirmed  by  P.  67. 

Procedure  57.  —  Detection  of  Chromium.  —  Acidify  the  filtrate 
from  the  Na2COs  precipitate  (P.  55)  with  HAc,  add  20  cc.  of 
water,  heat  the  mixture  nearly  to  boiling,  and  add  3-15  cc.  of 
PbAc2  solution.  (Yellow  precipitate,  presence  of  CHROMIUM.) 

Note.  —  The  solution  is  diluted  and  heated  nearly  to  boiling  so  as 
to  prevent  the  precipitation  of  PbCl2. 


102 


ANALYSIS  OF  IRON-GROUP 


P.  61 


ANALYSIS   OF  THE   IRON-GROUP 


TABLE  IX.  —  ANALYSIS  OF  THE  IRON-GROUP. 


Precipitate  Produced  by  Sodium  Hydroxide  and  Peroxide : 

A .  Phosphate  absent :  MnO(OH)2,  Fe(OH)3,*Zn(OH)2,  Co(OH)3, Ni(OH)2. 

B.  Phosphate  present:  Also  FePO-j,  and  alkaline-earth  phosphates  and 

carbonates. 
Heat  with  HN03  and  KClOs  (P.  61}. 


Precipitate  : 

Mn02. 
Add  HN03 


(P.  62). 


Solution : 

A.  Fe(N03)3,  Zn(N03)2)  Co(N03)2,  Ni(N03)2. 
Add  NH*OH(P.  63). 

B.  Also  Ba(N03)2)  etc.,  and  H3PO4. 

Nearly  neutralize  -with  NH*OH,  add  NH*Ac  and  Fe(NO3)a, 
dilute,  and  boil  (P.  64). \ 


Solution  : 
Mn(N03)2. 
AddBiOz 

Precipitate  : 
A.  Fe(OH)3. 
B.  Also  FePO4. 

Filtrate.  Add  NH^OH,  pass  inH2S  (P.  65). 

Precipitate  :  ZnS, 
CoS,  NiS. 
See  Table  X. 

Filtrate  : 
A.  NH4  salts.   Reject. 
B.  Ba,   Ca,   Sr,   Mg, 
and  NH4  salts. 
Treat  by  P.  71. 

Purple  color  : 
HMn04. 

*  All  the  zinc  may  be  carried  into  this  precipitate  by  elements  of  the  iron-group  when 
they  are  present  in  large  quantity. 

t  First  testing  a  small  portion  of  the  solution  for  iron  with  KjFe(CN)6. 

Procedure  61.  —  Precipitation  of  Manganese.  —  Transfer  the 
Na2O2  precipitate  (P.  52)  to  a  wide-mouth  50  cc.  conical  flask, 
and  add  5-10  cc.  of  16  n.  HNOs.  Remove  the  filter-paper  with 
a  glass  rod,  but  do  not  filter  out  any  undissolved  residue. 
Place  the  flask  in  a  beaker  of  boiling  water,  add  gradually  0.5 
cc.  of  powdered  KC103,  and  continue  to  heat  the  mixture  for 
1-2  minutes.  (Black  precipitate,  presence  of  MANGANESE.) 

In  case  there  is  no  precipitate,  treat  the  solution  by  P.  63 
if  in  P.  50  phosphate  was  found  absent,  or  by  P.  64  if  phosphate 
was  found  present. 

In  case  there  is  a  precipitate,  add  gradually  powdered  KC103, 
0.5  cc.  at  a  tune,  heating  (in  the  water-bath)  for  1-2  minutes 


P.  62  ANALYSIS  OF  IRON-GROUP  103 

after  each  addition,  till  no  further  precipitation  takes  place, 
not  adding  more  than  3  cc.  in  any  case.  Filter  the  mixture 
with  the  aid  of  suction  through  an  asbestos  filter  made  by  plac- 
ing in  a  funnel  a  2  cm.  perforated  plate  (or  enough  glass-wool  to 
form  a  wad  i  cm.  high,  tamping  it  down  with  the  finger),  and 
pouring  through  it  enough  of  a  suspension  of  fine  asbestos  fibers 
in  water  to  form  a  layer  of  asbestos  about  3  mm.  thick,  applying 
suction  at  the  same  time.  Collect  the  filtrate  in  a  test-tube 
placed  within  the  filter-bottle.  Wash  the  precipitate  with  a 
little  water,  and  treat  it  by  P.  62.  Treat  the  filtrate  by  P.  63 
if  in  P.  50  phosphate  was  found  absent,  or  by  P.  64  if  phosphate 
was  found  present. 

Notes.  —  i.  The  treatment  with  16  n.  HNOi  dissolves  all  the  sub- 
stances that  may  be  present  in  the  Na2O2  precipitate  except  MnO(OH)t ; 
and  this  may  be  dissolved  wholly  or  in  part  by  the  nitrous  acid  that  is 
present  in  the  HN03  or  is  produced  by  the  action  of  it  on  the  filter- 
paper.  Any  residue  of  MnO(OH)2  is  not  filtered  off  or  brought  into 
solution,  since  the  manganese  is  to  be  immediately  precipitated  as 
Mn02. 

2.  By  HClOa  in  hot  HNO,  solution  (but  not  by  HNOs  alone)  man- 
ganous  salts  are  rapidly  oxidized  to  MnO2  with  formation  of  ClOj 
(chlorine  dioxide),  which  escapes  as  a  yellow  gas. 

3.  The  KClOa  is  added  gradually,  so  as  to  avoid  too  violent  action 
in  case  much  manganese  is  present,  or  the  use  of  an  unnecessary  excess 
in  case  only  a  little  manganese  is  present.    The  mixture  is  heated  in  a 
water-bath,  instead  of  directly  over  a  flame,  in  order  to  avoid  the  risk 
of  the  acid  being  thrown  out  of  the  flask  by  bumping,  and  in  order  to 
obviate  the  danger  of  an  explosion,  which  might  occur  if  a  large  quantity 
of  C1O2  vapor  were  suddenly  produced  and  exposed  to  a  temperature 
above  100°.     Although  C1O2  is  highly  explosive,  having  a  great  tendency 
to  decompose  into  its  elements,  no  danger  is  involved  in  this  Procedure 
(where  the  mixture  is  heated  in  a  water-bath)  provided  a  large  quantity 
of  KClOa  be  not  added  at  one  time. 

4.  The  separation  of  manganese  in  this  way  from  the  other  metals 
of  this  group  is  entirely  satisfactory  with  the  exception  that  a  small 
quantity  of  iron  (up  to  i  mg.)  may  be  completely  carried  down  with  a 
large  quantity  (50x3  mg.)  of  manganese. 

Procedure   62.  —  Confirmatory    Test  for  Manganese.  —  Pour 
through  the  filter  containing  the  HC1O3  precipitate   (P.   61) 


104  ANALYSIS  OF  IRON-GROUP  P.6S 

5  cc.  of  hot  HN03  to  which  10  drops  of  3%  H202  solution  have 
been  added.  Collect  the  filtrate  in  a  test-tube;  cool  it;  add 
to  it  solid  bismuth  dioxide,  o.i  cc.  at  a  time,  till  a  purple  color 
results,  or  till  some  of  the  brown  solid  remains  undissolved ;  and 
let  the  solid  settle.  (Purple  solution,  presence  of  MANGANESE.) 

Notes.  —  i.  This  confirmatory  test  for  manganese  is  usually  super- 
fluous, since  the  precipitation  of  manganese  by  HClOj  is  highly  char- 
acteristic. 

2.  An  excess  of  bismuth  dioxide  must  be  added,  since  otherwise 
the  manganese  may  be  oxidized  only  to  MnOj. 

3.  Commercial  bismuth  dioxide,  also  often  called  sodium  bismuthate, 
is  a  mixture  of  bismuth  compounds  which  probably  owes  its  oxidizing 
power  to  the  presence  of  the  dioxide  BiOj.    When  it  is  not  available, 
PbOi  may  be  substituted  for  it ;  but  hi  that  case  the  mixture  must  be 
boiled  for  2  or  3  minutes. 

Procedure  63.  —  Precipitation  of  Iron  in  the  Absence  of  Phos- 
phate. —  In  case  phosphate  is  absent,  pour  the  cold  HN03 
solution  (P.  61),  all  at  once,  into  a  volume  of  6  n.  NH4OH  four 
tunes  as  large  as  that  of  the  16  n.  HNOs  used  in  P.  61.  (Dark 
red  precipitate,  presence  of  IRON.)  Filter,  and  wash  the  pre- 
cipitate. Treat  the  filtrate  by  P.  65.  Pour  on  to  the  filter 
containing  the  precipitate  2  cc.  of  K4Fe(CN)6  solution  to  which 
10  drops  of  HAc  have  been  added.  (Dark  blue  or  green  residue, 
presence  of  IRON.) 

Notes.  —  i.  The  HNO3  solution  is  poured  all  at  once  into  a  large 
excess  of  NH4OH;  for,  unless  the  iron  is  precipitated  very  rapidly 
from  a  solution  containing  a  large  quantity  of  ammonium  salts  and 
ammonia,  it  may  carry  down  with  it  nearly  all  the  cobalt  and  nickel, 
when  a  small  quantity  is  present.  Thus,  if  the  precipitation  is  made 
in  the  usual  way  by  adding  NH4OH,  a  little  at  a  time,  to  a  diluted 
HNOj  solution,  10  mg.  or  more  of  cobalt  and  2-3  mg.  of  nickel  are 
carried  down  by  500  mg.  of  iron  so  completely  that  these  elements  can 
hardly  be  detected  in  the  subsequent  Procedures;  while  with  the 
process  described  in  this  Procedure  10-20%  of  the  cobalt  and  about 
50%  of  the  nickel  pass  into  the  filtrate  when  small  quantities  of  them 
are  present  with  250-500  mg.  of  iron. 

2.  By  treatment  with  K4Fe(CN)6  the  Fe(OH)3  is  converted  into 
ferric  ferrocyanide  (Prussian  blue),  provided  some  HAc  be  present  so 
that  the  hydroxide  may  be  slightly  dissolved. 


P.  64  ANALYSIS  OF  IRON-GROUP  105 

Procedure  64.  —  Precipitation  of  Iron  and  Phosphate  in 
Presence  of  Phosphate.  —  Evaporate  one-tenth  of  the  HN03 
solution  (P.  61)  almost  to  dryness,  heat  the  residue  with  3  cc. 
of  HC1  till  the  odor  of  chlorine  disappears,  and  add  about  20  cc. 
of  water  and  3-30  drops  of  K4Fe(CN)6  solution.  (Dark  blue 
precipitate,  presence  of  IRON.) 

Neutralize  the  remainder  of  the  HNOs  solution  with  NH4OH, 
adding  it  toward  the  end  15  drops  at  a  time  till  a  precipitate 
forms  which  fails  to  redissolve  after  shaking  the  mixture  for 
10-15  seconds  (or  till  it  becomes  alkaline  in  case  there  is  no 
precipitate,  in  which  case  treat  the  mixture  directly  by  P.  65). 
Then  add  50  cc.  of  water  and  15  cc.  of  3  n.  NH4Ac  solution. 
Add  also,  unless  the  mixture  is  already  reddish  in  color,  Fe(NOs)3 
solution,  i  cc.  at  a  time,  till  such  a  color  is  produced.  Boil  the 
mixture  gently  for  2-3  minutes,  adding  30-50  cc.  more  water 
if  a  large  precipitate  separates.  (If  the  iron  does  not  precipitate, 
but  remains  in  the  colloidal  state,  add  10-30  drops  more  of 
NH4OH  and  boil  again.)  Filter  while  still  hot,  with  the  aid 
of  suction.  Reject  the  precipitate.  Make  the  nitrate  alkaline 
with  NH4OH ;  filter  out  any  precipitate,  and  treat  the  nitrate 
by  P.  65. 

Notes.  —  i .  This  separation  of  ferric  iron  from  the  bivalent  ele- 
ments depends  on  the  facts  that,  upon  boiling  an  acetic  acid  solution 
containing  much  acetate,  the  iron  is  completely  precipitated  as  Fe(OH)j 
and  Fe(OH)2Ac  (basic  ferric  acetate) ;  and  that  all  the  phosphate 
present  combines  with  the  iron  when  it  is  present  in  excess,  and  there- 
fore then  passes  completely  into  the  precipitate,  leaving  the  bivalent 
elements  in  solution. 

2.  The  precipitation  of  the  iron  takes  place  in  virtue  of  the  hy- 
drolysis (that  is,  the  decomposition  by  water)  of  the  ferric  acetate  into 
HAc  and  Fe(OH)s  or  Fe(OH)2Ac.  The  formation  of  Fe(OH)3  may  be 
expressed  by  the  equation : 

Fe++++3  Ac-+3  HOH=3  HAc+Fe(OH),. 

The  concentration  of  Fe(OH)s  evidently  tends,  in  accordance  with  the 
mass-action  law,  to  become  greater  (thereby  insuring  its  more  complete 
precipitation),  the  less  the  concentration  of  the  (unionized)  HAc  and 
the  greater  the  concentration  of  Ac~  ion.  For  this  reason  the  HNOj 
in  the  solution  is  carefully  neutralized  almost  completely,  and  a  large 


106  ANALYSIS  OF  IRON-GROUP  P.  65 

quantity  of  NH^Ac  is  added.  Some  free  HAc  must,  however,  be  left 
in  the  solution  and  an  undue  excess  of  NHiAc  must  not  be  present, 
since  otherwise  the  hydroxides  of  the  bivalent  elements  of  the  iron- 
group  would  precipitate  with  the  iron.  The  Fe(OH)3  formed  by  the 
hydrolysis  tends  to  remain  in  solution  hi  the  colloidal '  state.  Its 
coagulation  is  greatly  promoted  by  the  boiling  of  the  solution. 

3.  The  precipitation  of  the  phosphate  in  combination  with  the 
iron,  rather  than  with  the  alkaline-earth  elements,  depends  on  the  fact 
that  the  solubility  of  FePO4  in  water  (and  therefore  also  in  HAc)  is 
much  smaller  than  that  of  the  phosphates  of  the  bivalent  elements. 
The  ion-concentration  product  of  the  FePO4   therefore  attains  its 
saturation-value  and  removes  the  P04  from  the  solution  long  before 
the  ion-concentration  product  of  the  bivalent  phosphate  attains  its 
saturation-value. 

4.  In  order,  however,  that  the  alkaline-earth  phosphates  may  not 
be  precipitated  from  the  very  weak  HAc  solution,  more  than  enough 
iron  must  be  present  to  combine  with  all  the  phosphate  in  the  solution. 
If  this  amount  of  iron  is  already  present,  the  mixture  after  addition  of 
the  NH*Ac  will  have  a  reddish  color,  since  FeAc3  in  cold  solution  has  a 
deep  red  color.    If  this  amount  is  not  present,  the  mixture  will  be 
colorless  or  light  yellow  (unless  cobalt  or  nickel  is  present),  since  even 
if  there  is  some  iron  in  the  solution,  it  will  be  converted  into  FePO4 
which  forms  a  yellowish  white  precipitate.    In  this  case,  Fe(NO3)8  is 
added  till  the  mixture  becomes  reddish. 

Procedure  65.  —  Precipitation  of  Zinc,  Cobalt,  and  Nickel.  — 
Into  the  ammoniacal  solution  (P.  63  or  64)  pass  a  moderate 
current  of  H2S  for  about  a  minute  at  a  time,  till,  after  thorough 
shaking,  the  vapors  above  the  mixture  darken  PbAc2  paper. 
(White  precipitate,  presence  of  ZINC  ;  black  precipitate  or 
coloration,  presence  of  NICKEL  or  COBALT  ;  no  darkening  of  the 
mixture,  absence  of  NICKEL  and  COBALT.)  Filter,  and  wash  the 
precipitate  with  water  containing  i%  of  6  n.  (NH4)2S  solution. 
(In  case  the  nitrate  is  dark  colored,  boil  it  till  the  dark  color 
disappears,  and  filter  out  and  wash  any  precipitate,  uniting  it, 
if  dark-colored,  with  that  previously  obtained.)  Treat  the 
precipitate  by  P.  67  in  case  it  is  white,  or  by  P.  66  in  case  it  is 
dark-colored.  Treat  the  filtrate  by  P.  71-79  in  case  phosphate 
was  found  present  in  P.  50,  or  in  case  the  original  substance 
may  have  contained  oxalate  (see  Note  4) ;  treat  the  filtrate  by 


P.  65  ANALYSIS  OF  IRON-GROUP  107 

P.  78-79  in  case  chromium  was  found  present  in  P.  57 ;  other- 
wise, reject  the  nitrate. 

Notes.  —  i.  In  this  precipitation  HjS  is  used  instead  of  (NH4)»S, 
since  with  the  latter  reagent  much  of  the  nickel  may  remain  unpre- 
cipitated,  yielding  a  brown  solution.  Even  with  HjS  a  little  nickel 
may  pass  into  the  filtrate,  especially  if  a  large  excess  of  HjS  is  not 
avoided  by  shaking  the  mixture  and  testing  the  vapors  above  it  with 
PbAc2  paper  at  frequent  intervals. 

2.  Since  not  more  than  20  rag.  of  zinc  can  be  present  in  the  am- 
moniacal  solution  (see  Note  6,  P.  52),  and  since  i  mg.  of  nickel  or  cobalt 
causes  a  pronounced  darkening  of  such  a  small  quantity  of  ZnS,  it  is 
safe  to  conclude  that  nickel  and  cobalt  are  absent  when  no  such  darken- 
ing occurs,  and  the  further  Procedures  for  detecting  these  elements 
(P.  66  and  68)  may  be  omitted. 

3.  The  filtrate  must  be  tested  for  alkaline-earth  elements   (by 
P.  71-79)  in  case  phosphate  was  found  present  (in  P.  50),  or  in  case 
the  original  substance  may  have  contained  oxalate ;  for  these  elements 
may  then  be  found  wholly  or  in  large  part  in  this  filtrate,  as  explained 
in  Notes  6  and  7,  P.  51,  and  Note  i,  P.  64.     Since  the  filtrate  contains 
sodium  salts,  it  must  not  be  united  with  that  from  the  original  (NH4)2S 
precipitate  (obtained  in  P.  51),  but  must  be  separately  treated  by 
P.  71-79- 

4.  Oxalate,  like  other  organic  acidic  constituents,  is  never  present 
hi  minerals  or  alloys,  nor  hi  industrial  products  which   have   been 
subjected  to  a  high  temperature.    It  may,  however,  be  present  hi  other 
industrial  products ;    therefore,  when  the  original  substance  is  of  a 
character  and  from  a  source  that  makes  possible  the  presence  of  oxalate, 
the  filtrate  obtained  hi  the  Procedure  should  be  tested  for  alkaline- 
earth  elements.    These  elements  will  not  be  reprecipitated  by  NH4OH 
hi  P.  63  or  64,  but  will  pass  into  the  ammoniacal  filtrate,  since  the 
oxalic  acid  is  oxidized  to  CO2  and  H20  by  the  HC1O8  used  hi  P.  61. 

5.  The  filtrate  is  tested  for  magnesium  when  chromium  was  found 
present  in  P.  57,  since  any  magnesium  present  may  then  have  been 
completely  carried  down  with  it  in  the  (NH4)2S  precipitate,  as  stated 
in  Note  2,  P.  51. 


108  ANALYSIS  OF  IRON-GROUP  P. 

TABLE  X.  —  SEPARATION  OF  ZINC,  COBALT,  AND  NICKEL. 

Hydrogen  Sulphide  Precipitate :  ZnS,  CoS,  NiS. 
Treat  with  cold  i  n.  ECl  (P.  66). 


Solution:  ZnCl2,  CoCl2*,  NiCl2*. 

Add  NaOH  and  Na,O2  (P.  66). 


Filtrate: 

Add  (NH^S. 


White  precipitate :  ZnS. 
Dissolve   in  HN03, 
add  Co(NOz}z  and 

ignite  (P.  67). 


Green  residue :  CoZnO2. 


Residue : 
CoS,  NiS. 


Precipitate:  Co(OH)3,  Ni(OH)2. 

Add  HCl  and  KCIO*  (P.  68} . 

Splution :  CoCl2,  NiCl2. 

Evaporate,  add  HAc  and  KNO2  (P.  68). 


Yellow  precipitate : 
K3Co(N02)6. 


Filtrate:  NiCl2. 
Add  (CH3)2C2(NOn)z. 


Red  precipitate : 
[(CH3)2C2(NOH)NO]2Ni. 


*  A  small  proportion  of  the  cobalt  and  nickel  present  always  dissolves  in  the  dilute  HCl. 

Procedure  66.  —  Separation  of  Zinc  from  Cobalt  and  Nickel. 
—  Transfer  the  H2S  precipitate  (P.  65)  in  case  it  was  at  all 
darkened,  to  a  casserole.  Add  10-30  cc.  of  i  n.  HCl,  stir  the 
(cold)  mixture  frequently  for  5  minutes,  and  filter  it.  (Black 
residue,  presence  of  COBALT  or  NICKEL.)  Wash  the  residue  and 
treat  it  by  P.  68  (after  uniting  it  with  the  Na202  precipitate 
obtained  from  the  nitrate).  Boil  the  filtrate  till  the  H2S  is 
completely  expelled,  make  the  mixture  alkaline  with  NaOH 
solution,  cool  it,  and  add  gradually  0.5-1.0  cc.  of  Na202  powder. 
Boil  the  mixture  gently  for  2-3  minutes,  cool  it,  and  filter  it. 
(Black  precipitate,  presence  of  COBALT  ;  green  precipitate,  pres- 
ence of  NICKEL.)  Wash  the  precipitate,  unite  it  with  the  residue 
undissolved  by  dilute  HCl,  and  treat  the  mixture  by  P.  68. 
To  the  filtrate  add  3-10  drops  of  6  n.  (NH4)2S  solution.  (White 
precipitate,  presence  of  ZINC.)  Filter  out  and  wash  the  pre- 
cipitate, and  treat  it  by  P.  67. 


P.  66  ANALYSIS  OF  IRON-GROUP  109 

Notes.  —  i.  This  treatment  with  i  n.  HC1  serves  to  extract  from 
the  cobalt  and  nickel  sulfides  nearly  all  the  zinc  which  may  be  present 
in  this  precipitate  because  of  its  having  been  carried  down  in  the  Na2O2 
precipitate,  as  described  in  Note  6,  P.  52.  A  small  proportion  of  the 
cobalt  and  nickel  present  (5-20%)  always  dissolves  in  the  in.  HC1, 
and  the  subsequent  treatment  with  Na2O2  serves  to  separate  these 
elements  from  the  zinc.  This  Na202  separation  is  satisfactory  when, 
as  in  this  HC1  solution,  the  cobalt  and  nickel  are  present  in  small 
quantity ;  for  then  only  an  insignificant  amount  of  zinc  is  carried  down 
with  them. 

2.  This  Procedure  must  always  be  followed  in  order  to  determine 
whether  or  not  zinc  is  present  in  the  substance,  unless  a  satisfactory 
test  for  it  has  already  been  obtained  in  P.  56,  or  unless  the  original 
Na202  precipitate  (P.  52)  was  small.     In  either  of  these  two  cases  this 
Procedure  may  be  omitted  and  the  H2S  precipitate  (P.  65)  treated 
directly  by  P.  68. 

3.  The  fact  that  CoS  and  NiS  do  not  dissolve  readily  in  i  n.  HC1 
seems  inconsistent  with  the  non-precipitation  of  cobalt  and  nickel  by 
HjS  with  the  copper-  and  tin-groups  from  a  solution  which  is  only  0.3 
normal  in  acid.     This  behavior  probably  arises  from  the  fact  that  these 
sulfides  exist  in  at  least  two  allotropic  forms  of  different  solubilities. 
The  form  that  is  first  produced  when  sulfide-ion  is  brought  together 
with  nickel-ion  or  cobalt-ion  is  soluble  in  dilute  acid ;   but  this  form 
changes  on  standing  or  heating  into  a  less  soluble  form,  which  is  not 
produced  directly  except  when  the  ion-constituents  are  mixed  at  fairly 
high  concentrations.     This  would  explain  also  the  fact  that  the  CoS  and 
NiS  precipitated  by  (NH4)2S  dissolve  partly,  but  not  wholly,  in  dilute 
HC1 ;    for  there  are  doubtless  present  in  these  precipitates  both  the 
more  soluble  and  the  less  soluble  forms  —  the  latter  in  larger  propor- 
tion the  longer  the  precipitates  have  been  heated  and  the  longer  they 
have  stood. 

4.  NaOH  precipitates  cobalt  as  blue  CoCl(OH),  changing  to  pink 
Co(OH)2,  and  nickel  as  light-green  Ni(OH)2.     By  the  addition  of 
NajOii,  the  cobalt  is  oxidized  to  black  Co(OH)3,  and  its  precipitation 
made  more  complete. 

5.  Only  a  small  quantity  (0.5-1.0  cc.)  of  Na2O2  powder  is  added, 
since  the  quantity  of  cobalt  and  nickel  dissolved  by  the  dilute  HC1 
never  exceeds  100  mg.    The  solution  is  subsequently  boiled  to  decom- 
pose the  excess  of  Na202,  since  otherwise  it  would  destroy 

added  to  precipitate  the  zinc. 


no  ANALYSIS  OF  IRON-GROUP  P.  68 

Procedure  67.  —  Confirmatory  Test  for  Zinc.  —  Dissolve  the 
H2S  precipitate  (P.  65)  in  case  it  was  white,  or  the  (NH^zS 
precipitate  (P.  67),  by  pouring  a  5-10  cc.  portion  of  HNO3 
repeatedly  through  the  filter.  To  the  solution  add  from  a 
dropper  1-3  drops  of  0.3  n.  Co(N03)2  solution.  Evaporate  the 
mixture  just  to  dryness,  and  add  1-3  cc.  of  3  n.  Na2CO3  solution. 
Evaporate  again  to  dryness,  and  ignite  the  residue  at  a  low 
temperature  by  keeping  the  dish  moving  to  and  fro  in  a  small 
flame  till  the  purple  color  due  to  the  cobalt  disappears ;  let  the 
casserole  cool,  and  moisten  the  residue  with  water.  (Green 
residue,  presence  of  ZINC.)  (If  the  ignited  mass  becomes  black, 
owing  to  too  strong  heating,  add  a  few  drops  of  HNO3,  evapo- 
rate just  to  dryness,  add  the  same  quantity  of  Na2C03  solution 
as  was  added  before,  evaporate  and  ignite  as  before.) 

Notes.  —  i.  This  confirmatory  test  is  useful  when  there  results  only 
a  small  noncoagulating  precipitate  which  may  be  sulfur,  or  when, 
owing  to  the  presence  of  a  small  quantity  of  other  elements,  the  pre- 
cipitate is  dark-colored. 

2.  In  this  test  the  use  of  an  excess  of  Co(NOj)2  is  avoided,  since 
otherwise  the  black  color  of  the  CoO  obscures  the  green  color  of  the 
cobalt  zincate  (CoZnOz) ;  and  for  this  reason  the  amount  of  Co(NOj), 
added  is  adjusted  within  the  limits  stated  to  the  quantity  of  the  pre- 
cipitate tested.  The  test  when  properly  made  will  detect  0.5  mg.  or 
less  of  zinc. 

Procedure  68.  —  Detection  of  Cobalt  and  Nickel.  —  Dissolve 
the  residue  from  the  HC1  treatment  and  the  Na^Oz  precipitate 
(P.  66)  in  a  casserole  by  adding  5-15  cc.  of  HC1,  heating  the 
mixture  nearly  to  boiling,  and  sprinkling  into  it  0.1-0.3  cc.  of 
powdered  KC103.  Filter  out  any  sulfur  residue,  and  evaporate 
the  solution  just  to  dryness.  Dissolve  the  residue  in  5  cc.  of 
HAc.  To  the  solution  in  a  test-tube  add  3  cc.  of  6  n.  KNO2 
solution,  and  let  the  mixture  stand  with  occasional  shaking  for 
at  least  15  minutes.  (Yellow  precipitate,  presence  of  COBALT.) 
If  a  considerable  precipitate  forms,  add  to  the  mixture  10  cc. 
more  of  6  n.  KNO2  solution  and  4  cc.  of  powdered  KC1,  and 
let  it  stand  with  frequent  shaking  for  at  least  15  minutes.  Filter. 
Reject  the  precipitate.  To  one-fourth  of  the  filtrate  add  10  cc. 


P.  68  ANALYSIS  OF  IRON-CROUP  in 

of  water  and  4  cc.  of  a  o.i  n.  dime  thy  Iglyoxime  solution  in 
ethylalcohol,  heat  the  mixture  nearly  to  boiling,  and  let  it  stand 
5-10  minutes.  (Red  precipitate,  presence  of  NICKEL.)  Reject 
the  remainder  of  the  filtrate. 

Notes.  — i.  Only  a  small  quantity  (3  cc.)  of  KNO,  solution  is 
added  at  first,  since  in  the  small  volume  of  solution  this  suffices  to  give 
a  distinct  precipitate  with  less  than  i  mg.  of  cobalt  within  15  minutes, 
and  since  with  this  small  quantity  there  is  no  danger  that  nickel  will 
be  precipitated.  In  case  a  considerable  precipitate  forms,  a  much 
larger  quantity  of  the  KNOZ  reagent  is  added,  together  with  enough 
solid  KC1  to  nearly  saturate  the  solution,  in  order  to  precipitate  nearly 
all  of  the  cobalt ;  for  the  presence  of  much  cobalt  interferes  with  the 
subsequent  test  for  nickel.  Even  though  under  these  conditions  some 
nickel  may  be  precipitated,  especially  when  much  of  it  is  present  to- 
gether with  a  large  quantity  of  cobalt,  only  a  small  proportion  of  the 
nickel  present  is  ever  carried  down. 

2.  The  principles  involved  in  this  separation  of  cobalt  from  nickel 
are  as  follows.     A  small  proportion  of  the  cobalt  present  is  oxidized 
by  the  HNO2  from  the  cobaltous  to  the  cobaltic  state  (from  the  state 
of  Co"1"1"  to  Co+++  ions).    The  value  of  the  reduction-potential  for 
Co++,  Co+++,  is,  however,  such  that  the  oxidation  must  soon  cease 
unless  the  Co+++  ions  are  removed  from  the  solution.    This  is  effected 
in  this  case  by  the  KNO2  through  the  conversion  of  the  Co+++  ions 
into  the  complex  anion  Co(NO2)«=  and  through  the  precipitation  of  the 
latter  in  the  form  of  the  slightly  soluble  K3Co(NOj)»  (potassium  co- 
baltinitrite).    It  is  clear  from  these  statements  that,  in  accordance  with 
the  mass-action  law,  a  large  concentration  of  HNOj  will  hasten  the 
oxidation  of  the  cobaltous  ion,  that  a  large  concentration  of  N02~  ion 
will  cause  more  complete  conversion  of  the  cobaltic  ion  into  the  co- 
baltinitrite  ion,  and  that  a  large  concentration  of  K+  ion  will  diminish 
the  solubility  of  the  potassium  cobaltinitrite.    It  will  be  seen  that  all 
these  conditions  are  provided  for  in  the  Procedure.    The  nickel  is  not 
appreciably  oxidized  by  HNOZ  to  the  nickelic  state.    When  it  is  pre- 
cipitated by  KNO2,  it  is  in  the  form  of  the  (fairly  soluble)  potassium 
nickelous  nitrite  K4Ni(N02)«. 

3.  Only  one-fourth  of  the  filtrate  is  used  for  the  nickel  test  with 
dimethylglyoxime,  since  this  suffices  to  give  the  desired  delicacy,  since 
it  enables  the  quantity  of  nickel  present  to  be  better  estimated,  and 
since  it  diminishes  the  quantity  of  cobalt  present  in  the  solution  tested. 
It  is  desirable  that  not  much  cobalt  be  present,  since  a  smaller  volume 
of  the  (rather  expensive)  reagent  is  then  required ;  for  a  quantity  of 


H2  ANALYSIS  OF  IRON-GROUP  P.  68 

the  dimethylglyoxime  equivalent  to  the  cobalt  present  must  be  added 
before  a  small  quantity  of  nickel  will  yield  a  precipitate.  This  is 
probably  due  to  the  fact  that  the  cobalt  combines  with  the  reagent 
forming  a  soluble  complex  salt. 

4.  Dimethylglyoxime  is  a  weak  monobasic  organic  acid  of  the  com- 
position (CHS)2C2(NOH)2.  The  brilliant  red  substance  is  its  nickel 
salt,  formed  by  replacing  by  one  nickel  atom  one  hydrogen  atom  hi 
each  of  two  dimethylglyoxime  molecules.  The  precipitate  is  so  slightly 
soluble,  so  voluminous,  and  so  highly  colored  that  less  than  o.i  mg.  of 
nickel  can  be  detected  in  the  solution  tested. 


P.  71       PRECIPITATION  OF  ALKALINE-EARTH  GROUP       113 
PRECIPITATION   AND   ANALYSIS   OF   THE   ALKALINE-EARTH   GROUP 


TABLE  XI.  —  ANALYSIS  OF  THE  ALKALINE-EARTH  GROUP. 


Ammonium  Carbonate  Precipitate : 

BaCO3,  SrC03,  CaCOj,  MgCO3  •  (NH4)2C03. 

Dissolve  in  HAc,  add  NH4Ac  and  K£rO4  (P.  72). 


Precipitate  : 
BaCr04. 
Dissolve  in  HCl, 
evaporate,  add 
HAc,  NHtAc, 
and  K£r04 
(P.  73). 

Filtrate.    Add  NH,OH  and  C^H,OH  (P.  74). 

Precipitate  : 
SrCr04,  (CaCrO4).* 
Boil  with  (NH^Ot 
and  K££>4,  (P.  75). 

Filtrate  :   Ca  and  Mg  salts. 
Add  K££><  (P.  76). 

Precipitate  : 
CaC204,  (MgC204). 
Treat  with  HzSOt. 

Filtrate. 
Add 
NatHPOi 
(P.  78). 

Residue  : 
SrC03,  (CaC204). 
Treat  with  HAc. 

Precipitate  : 
BaCrO4. 

Solution  : 
CaS04,  (MgS04). 
Add  C^tOH 
(P.  77). 

Precipitate  : 
MgNH4PO4. 

Solution  : 
SrAc2. 
Add  NaiSOi. 

Residue  : 
(CaC204). 

Precipitate  : 
CaS04. 

Precipitate  : 
SrS04. 

*  Substances  whose  formulas  are  within  parentheses  are  not  normally  found  at  the 
point  indicated,  but  their  presence  (arising  from  faulty  procedure  or  an  excessive  propor- 
tion of  the  element  in  the  substance)  is  provided  for  in  the  confirmatory  tests. 

Procedure  71.  —  Precipitation  of  the  Alkaline-Earth  Group.  — 
Evaporate  the  filtrate  from  the  (NH4)2S  precipitate  (P.  51)  till 
salts  crystallize  out,  dilute  it  to  a  volume  of  10  cc.,  cool  it,  and 
filter  out  any  sulphur  or  crystalline  salts  that  have  separated. 

To  the  filtrate  add  15  cc.  of  9  n.  (NH4)2CO3  solution  and 
15  cc.  of  95%  C2H5OH  (ethyl  alcohol) ;  and,  if  a  large  precipitate 
results,  add  15  cc.  more  of  each  of  these  liquids.  Shake  the 
mixture  continuously  for  10  minutes;  or  better,  let  it  stand, 
with  frequent  shaking,  for  at  least  half  an  hour.  (Precipitate, 
presence  of  ALKALINE-EARTH  ELEMENTS.)  Filter,  and  wash  the 
precipitate  with  a  little  (NH4)2CO3  reagent,  using  suction  if 


114  ANALYSIS  OF  ALKALINE-EARTH  GROUP  P.  72 

the  precipitate  is  large.  Treat  the  precipitate  by  P.  72.  Treat 
the  filtrate  by  P.  81  or  P.  85  (see  the  General  Discussion  of 
the  Alkali- Group  on  page  121). 

Notes.  —  i.  The  filtrate  from  the  (NH^S  precipitate  is  evaporated 
in  order  that  the  elements  of  the  alkaline-earth  group  may  be  pre- 
cipitated more  quickly  and  more  completely.  The  evaporation  also 
serves  to  destroy  (NH^S  and  to  coagulate  any  sulfur  that  may  separate. 
The  volume  to  which  the  (NH^COs  reagent  is  added  should  be  10  cc. 

2.  If  the  ammonium  carbonate  solution  were  added  in  only  small 
excess,  the  precipitation  of  CaCO3,  SrCO3,  and  BaCO3  would  not  be 
complete,  and  additional  tests  for  small  quantities  of  these  elements 
would  have  to  be  made  in  the  nitrate.    But,  by  the  use  of  a  large 
quantity  of  a  concentrated  solution  of  (NH4)2CO3  containing  NHjOH 
(so  as  to  diminish  the  hydrolysis  of  the  carbonate  into  (NH^+HCOs" 
and  NH4OH),  the  precipitation  may  be  made  practically  complete, 
owing  to  the  large  concentration  of  carbonate-ion  (C03=). 

3.  By  making,   as  in   the   Procedure,   the   concentration  of   the 
(NH^COs  sufficiently  great,  magnesium  is  in  the  cold  also  completely 
precipitated.    The  precipitate,  which  is  in  this  case  a  double  carbonate, 
MgC03-(NH4)2CO3-4H20,  is,  however,  fairly  soluble  in  cold  water 
and  readily  soluble  in  hot  water. 

4.  From  a  cold  aqueous  solution  the  precipitation  of  these  elements 
takes  place  slowly,  especially  in  the  case  of  magnesium  and  calcium; 
but  it  is  greatly  accelerated  by  the  addition  of  alcohol  and  by  shaking. 
Under  the  conditions  recommended  in  the  procedure  0.5  mg.  of  any 
of  the  four  elements  gives  a  precipitate. 

Procedure  72.  —  Precipitation  of  Barium.  —  Pour  repeatedly 
through  the  filter  containing  the  (NH4)2CO3  precipitate  (P.  71) 
a  5-15  cc.  portion  of  hot  HAc,  and  evaporate  the  solution  just 
to  dryness,  taking  care  not  to  ignite  the  residue. 

Add  to  the  residue  2  cc.  of  HAc,  10  cc.  of  3  n.  NH4Ac  solution, 
and  10  cc.  of  water,  and  heat  the  mixture  nearly  to  boiling  in  a 
flask.  Add  just  3  cc.  of  3  n.  K2Cr04  solution,  5  drops  at  a  time, 
shaking  after  each  addition,  and  heat  the  mixture  nearly  to 
boiling  for  5  minutes,  with  frequent  shaking.  (Yellow  precipi- 
tate, presence  of  BARIUM.)  In  case  a  considerable  precipitate 
forms,  add  2  cc.  more  of  the  K2CrO4  solution,  and  heat  and  shake 
the  mixture.  Filter,  even  though  the  solution  appear  clear. 
(Precipitate  P.  73 ;  filtrate  P.  74.) 


P.  78          ANALYSIS  OF  ALKALINE-EARTH  GROUP  11$ 

Notes.  —  i.  The  separation  of  the  first  three  elements  of  this  group 
depends  on  the  difference  in  the  solubilities  of  their  chromates. 
These  solubilities  increase  rapidly  in  the  order,  Ba,  Sr,  Ca,  as  shown 
in  the  Table  in  the  Appendix.  The  difference  in  the  solubilities  of 
BaCrQj  and  SrCrO4  is  so  great  that  under  the  conditions  of  the  pro- 
cedure 0.5  mg.  Ba  can  be  detected,  while  500  mg.  of  strontium  give  no 
precipitate,  even  when  5  cc.  of  K2CrO4  solution  have  been  added. 

2.  In  case  a  large  quantity  of  barium  is  present,  a  second  portion 
of  the  K2CrO4  reagent  is  added ;  for  though  the  quantity  of  K2CrO4 
present  in  the  3  cc.  first  added  is  equivalent  to  about  600  mg.  of  barium, 
yet  the  excess  left  in  the  solution  when  much  barium  is  present  may  be 
so  small  as  not  to  precipitate  it  completely.    A  larger  amount  of 
K2CrO4  is  not  added  at  first,  and  it  is  not  added  at  all  unless  the  presence 
of  much  barium  makes  it  necessary,  since  it  would  increase  the  tendency 
of  strontium  (and  in  P.  74  of  calcium)  to  precipitate. 

3.  Acetic  acid  is  added  to  increase  the  solubility  of  SrCrO4.    By  its 
action  the  concentration  of  the  chromate-ion  is  decreased,  owing  to  its 
conversion  partly  into  hydrochromate-ion  and  partly  into  bichromate- 
ion,  according  to  the  reactions : 

Cr04=+H+=HCr04-;  and  2  HCrOr=H20+Cr207=. 
It  is  evident  that  the  ratio  of  the  CrO4~  to  the  HCrO4~  concentration 
must  decrease  as  the  H+  concentration  increases.  For  this  reason  the 
presence  of  a  largely  ionized  acid  (such  as  HC1  or  HN03)  would  prevent 
the  complete  precipitation  of  BaCrO4 ;  but  since  HAc  is  only  a  slightly 
ionized  acid,  and  since  a  large  amount  of  acetate  is  present,  the  addition 
of  a  considerable  quantity  of  HAc  has  only  a  slight  effect  on  the  solu- 
bility. 

4.  The  K2Cr04  is  added  slowly  to  the  hot  solution  and  the  mixture 
is  shaken  and  heated  in  the  neighborhood  of  100°  before  filtering,  since 
otherwise  the  precipitate  is  liable  to  pass  through  the  filter. 

5.  By  adding  the  reagent  gradually  almost  all  the  barium  is  pre- 
cipitated before  an  excess  of  K2CrO4  is  present.    This  is  of  importance, 
since,  when  much  barium  is  present,  even  3  mg.  of  strontium  may  be 
carried  down  completely  if  the  K2CrO4  reagent  be  added  quickly. 

6.  When  less  than  i  mg.  of  barium  is  present,  it  is  difficult  to  dis- 
tinguish the  faint  turbidity  in  the  colored  solution.    It  is  therefore 
directed  to  filter  the  mixture  even  when  it  appears  clear;  for  a  very 
small  yellow  precipitate  can  be  seen  on  the  filter  after  washing  out  the 
K2Cr04  solution. 

Procedure  73.  —  Confirmatory  Test  for  Barium.  —  In  case  in 
P.  74  much  strontium  is  found  present,  pour  repeatedly  through 


n6  ANALYSIS  OF  ALKALINE-EARTH  GROUP          P.  74 

the  filter  containing  the  K2Cr04  precipitate  (P.  73)  a  5-10  cc.  por- 
tion of  hot  HC1,  and  evaporate  the  solution  just  to  dryness.  Treat 
the  residue  as  described  in  the  second  paragraph  of  P.  72,  using 
however  only  one-half  of  the  prescribed  volume  of  each  of  the 
reagents.  (Yellow  precipitate,  presence  of  BARIUM.) 

Note.  —  In  case  a  large  quantity  of  strontium  is  present,  as  shown 
by  the  formation  of  a  large  precipitate  in  P.  74,  some  of  it  may  be 
precipitated  by  K2Cr04  in  P.  72,  especially  if  the  directions  as  to  the 
quantities  of  the  reagents  have  not  been  followed.  The  strontium  will 
not,  however,  again  precipitate  in  this  second  treatment ;  for  the 
quantity  of  it  now  present  in  the  solution  is  much  less  than  before. 
A  yellow  precipitate  now  obtained  is  therefore  conclusive  evidence  of 
the  presence  of  barium. 

Procedure  74.  —  Precipitation  of  Strontium.  —  To  the  filtrate 
from  the  K2Cr04  precipitate  (P.  72)  add  NH4OH  slowly  till 
the  color  changes  from  orange  to  yellow,  and  then  5  cc.  more. 
Heat  the  solution  to  60-70°,  and  add  to  it  15  cc.  of  95%  C2H5OH 
5  cc.  at  a  time,  shaking  for  10-15  seconds  after  each  addition  if 
a  precipitate  forms.  Cool  the  solution  in  running  water,  shaking 
it  continuously;  and  let  the  mixture  stand  at  least  5  minutes. 
(Light  yellow  precipitate,  presence  of  STRONTIUM.)  In  case  con- 
siderable precipitate  forms,  add  5  cc.  of  3  n.  K2CrO4  solution 
and  15  cc.  of  95%  C2H5OH,  shake  the  mixture  and  let  it  stand 
at  least  5  minutes.  Filter  with  the  aid  of  suction  (see  Note  4) ; 
suck  the  precipitate  as  dry  as  possible,  but  do  not  wash  it.  (Pre- 
cipitate, P.  75 ;  filtrate,  P.  76.) 

Notes.  —  i.  Under  these  conditions  i  mg.  of  strontium  yields  a 
precipitate,  and  500  mg.  of  calcium  or  magnesium  do  not  do  so,  pro- 
vided not  more  than  3  cc.  of  K2CrO4  solution  and  15  cc.  of  C2HBOH 
have  been  added.  Upon  the  addition  of  the  second  portions  of  these 
reagents,  a  precipitate  of  CaCrCX  may  separate  when  a  large  quantity 
of  calcium  is  present.  This  does  not  interfere  with  the  detection  of 
strontium ;  for  these  additional  amounts  of  reagent  are  added,  in  order 
to  precipitate  completely  a  large  quantity  of  strontium,  only  when 
the  smaller  amounts  have  already  produced  a  considerable  precipitate. 

2.  Since  calcium  may  precipitate  even  at  first  if  the  concentrations 
of  the  reagents  differ  much  from  those  prescribed,  the  confirmatory 
test  for  strontium  should  always  be  tried 


P.  75  ANALYSIS  OF  ALKALINE-EARTH  GROUP  117 

3.  The  alcohol  is  added  to  the  hot  solution,  in  small  portions,  and 
with  vigorous  shaking,  so  that  all  the  SrCr04  may  not  be  suddenly 
precipitated,  which  may  cause  it  to  separate  in  so  fine  a  form  that  it 
passes  through  the  pores  of  the  filter-paper. 

4.  If  the  filtrate  is  not  perfectly  clear,  even  after  a  second  filtration, 
it  may  be  made  so  by  adding  some  paper-pulp,  shaking  vigorously 
for  a  minute  or  two,  and  again  filtering  with  the  aid  of  suction.    The 
pulp  may  be  made  by  shaking  violently  a  filter-paper  torn  into  small 
pieces  with  10  cc.  of  water  in  a  stoppered  test-tube,  and  pressing  out 
most  of  the  water  between  the  fingers. 

5.  The  precipitate  is  not  washed,  since  SrCrO4  is  fairly  soluble  in 
water. 

Procedure  75.  —  Confirmatory  Test  for  Strontium.  —  Pour  re- 
peatedly through  the  filter  containing  the  K2CrO4  precipitate 
(P. 74)  a  IQ-CC.  portion  of  boiling  water.  Add  to  the  solution  just 
i  cc.  of  3  n.  Na2CO3  solution  and  1 2  cc.  of  3  n.  K2C2O4  solution, 
and  boil  the  mixture  gently  in  a  covered  casserole  for  5  minutes. 
Filter  the  boiling  mixture.  Reject  the  filtrate.  Wash  the  pre- 
cipitate thoroughly  with  water,  and  pour  repeatedly  through 
the  filter  5  cc.  of  cold  i  n.  HAc.  To  the  solution  add  2  cc.  of 
Na2SO4  solution,  heat  the  mixture  to  boiling,  and  let  it  stand 
10  minutes.  (White  precipitate,  presence  of  STRONTIUM.) 

Notes.  —  i.  In  a  boiling  solution  containing  oxalate  and  carbonate 
in  the  proportions  recommended  in  the  Procedure  small  quantities  of 
strontium  and  calcium  are  converted  in  a  few  minutes  almost  com- 
pletely into  SrCOs  and  CaC2O4,  respectively.  This  behavior  arises 
from  the  facts  that  in  the  hot  mixture  SrCOs  is  less  soluble  than  SrC2O4, 
while  CaC204  is  less  soluble  than  CaC03. 

2.  The  mixture  is  filtered  while  still  near  the  boiling  temperature; 
for  in  the  cold  the  solubility  relations  are  such  that  SrC204  instead 
of  SrCOs  results. 

3.  SrC03  is  readily  dissolved  by  i  n.  HAc,  but  CaC2O4  is  only 
slightly  dissolved  by  this  acid.    Any  small  quantity  of  calcium  that 
passes  into  solution  would  not  give  a  precipitate  with  Na2S04,  since 
CaS04  is  a  moderately  soluble  salt. 

4.  Of  any  barium  that  may  be  contained  in  the  K2CrO4  precipitate 
only  enough  is  dissolved  by  the  boiling  water  (in  the  presence  of  the 
K2CrO4  and  NHiOH  remaining  in  the  filter-paper)  to  give  a  scarcely 
noticeable  turbidity  with  the  Na2S04  solution. 


u8  ANALYSIS  OF  ALKALINE-EARTH  GROUP  P.  76 

Procedure  76.  —  Precipitation  of  Calcium.  —  Dilute  the  am- 
moniacal  filtrate  from  the  K2CrO4  precipitate  (P.  74)  with  50  cc. 
of  water,  add  just  3  cc.  of  3  n.  K2C2O4  solution,  and,  unless  there 
is  already  a  precipitate,  let  the  mixture  stand  at  least  15  minutes. 
(Fine  white  precipitate,  presence  of  CALCIUM.) 

In  case  there  is  no  precipitate,  treat  the  solution  by  P.  78. 

In  'case  there  is  a  precipitate,  heat  the  mixture  nearly  to  boil- 
ing, and  add  gradually  3-10  cc.  more  3  n.  K2C2O4  solution,  ad- 
justing the  volume  added  to  the  size  of  the  (NH4)2CO3  precipitate 
produced  in  P.  71.  Continue  to  heat  the  mixture  for  5  minutes ; 
then  filter  it  immediately,  and  wash  the  precipitate.  (Precipi- 
tate, P.  77 ;  filtrate,  P.  78.) 

Notes.  —  i.  Only  3  cc.  of  the  K2C204  solution  are  added  at  first, 
in  order  to  prevent,  so  "far  as  is  possible,  the  precipitation  of  MgC2O4. 
This  volume  of  reagent  would  not  precipitate  magnesium  unless  there 
were  present  a  very  large  quantity  of  it  (more  than  300  mg.),  such  as 
would  very  rarely  be  found  (in  view  of  the  small  equivalent  weight  of 
this  element)  in  the  one  gram  of  substance  taken  for  analysis.  But  if  a 
larger  volume  of  the  reagent  were  used,  a  precipitate  might  result  with 
a  smaller  quantity  of  magnesium. 

2.  With  even  this  small  quantity  of  the  reagent  £  mg.  of  calcium 
gives  an  almost  immediate  precipitate  when  not  much  magnesium  is 
present ;  but  in  the  presence  of  a  large  quantity  of  magnesium  the  test 
is  much  less  delicate.    Thus,  in  order  to  detect  i  mg.  of  calcium  in 
the  presence  of  300  mg.  of  magnesium,  the  mixture  must  be  allowed  to 
stand  for  ten  to  fifteen  minutes. 

3.  To  insure  the  complete  precipitation  of  calcium,  a  larger  volume 
(3-10  cc.)  of  the  K2C2O4  reagent  is  subsequently  added ;  and,  though 
this  may  cause  the  precipitation  of  some  of  the  magnesium,  this  is  not 
important  since  the  presence  or  absence  of  calcium  has  already  been 
determined. 

4.  The  volume  of  K2C204  solution  finally  added  should  be  adjusted, 
not  as  usual  to  the  size  of  the  precipitate  that  the  reagent  produces, 
but  to  the  sum  of  the  quantities  of  calcium  and  magnesium  that  seem 
to  be  present ;  and  since  the  only  indication  at  this  stage  of  the  analysis 
as  to  the  magnitude  of  these  quantities  is  that  afforded  by  the  original 
(NH4)2C03  precipitate  (considered  in  connection  with  the  quantities 
of  barium  and  strontium  already  found  present),  it  is  directed  to  adjust 
the  added  volume  of  the  reagent  (within  the  limits  of  3-10  cc.)  to  the 
•yze  of  that  precipitate. 


P.  77  ANALYSIS  OF  ALKALINE-EARTH  GROUP  119 

5.  The  peculiar  influence  of  magnesium  in  hindering  the  precipita- 
tion of  CaC204  arises  from  the  fact  that  MgC2O4  is  far  less  ionized  than 
most  other  salts  of  the  same  valence  type,  and  that  in  consequence  the 
C2O4~  ion  of  the  reagent  largely  combines  with  the  Mg++  ion  in  the 
solution  until  there  has  been  added  a  quantity  of  K2C204  more  than 
equivalent  to  the  magnesium  present. 

6.  The  mixture  is  heated  to  boiling  and  the  K2C204  solution  is  added 
slowly,  in  order  to  cause  the  CaC204  to  precipitate  in  the  form  of  coarser 
particles  which  can  be  more  readily  filtered.    The  mixture  is  kept  hot 
for  5  minutes  to  insure  the  complete  precipitation  of  calcium. 

7.  The  mixture  is  not  heated  for  more  than  5  minutes  and  is  filtered 
immediately,  in  order  to  prevent  so  far  as  possible  the  precipitation  of 
MgC204  •  2  H2O.    This  substance,  though  slightly  soluble  in  water, 
has  an  unusually  great  tendency  to  remain  in  supersaturated  solution, 
especially  if  agitation  of  the  solution  be  avoided. 

8.  Owing  to  the  possibility  that  the  precipitate  formed  on  the  first 
addition  of  K2C2O4  may  consist  of  magnesium  or  strontium  oxalate,  the 
confirmatory  test  for  calcium  given  hi  P.  77  should  not  be  omitted. 

Procedure  77.  —  Confirmatory  Test  for  Calcium.  —  Treat  the 
K2C2O4  precipitate  (P.  76),  or  a  small  portion  of  it  if  it  is  large, 
with  5  cc.  of  H2SO4  to  which  20  drops  of  C2H5OH  have  been  added. 
To  the  solution  add  10  cc.  of  C2H5OH,  and  let  the  mixture  stand 
for  several  minutes.  (White  precipitate,  presence  of  CALCIUM.) 

Notes.  —  i.  CaC204  •  H20  is  very  slightly  soluble  in  water,  but 
reacts  with  dilute  solutions  of  largely  ionized  acids,  owing  to  the  forma- 
tion by  metathesis  of  unionized  HC2O4~.  Because  of  its  slight  solu- 
bility, only  a  little  CaSO4  dissolves  in  the  dilute  H2SO4,  but  this  is 
completely  thrown  out  as  a  flocculent  precipitate  by  the  addition  to 
the  solution  of  twice  its  volume  of  C2H5OH.  One  milligram  of  cal- 
cium produces  a  turbidity  at  once,  0.5  mg.  in  1-3  minutes,  and  0.2  mg. 
within  10  minutes.  This  test  does  not,  however,  furnish  any  indica- 
tion of  the  quantity  of  calcium  present  except  when  this  is  very  small ; 
for  whatever  that  quantity,  not  more  than  about  i  mg.  of  calcium 
dissolves  in  the  mixture  of  5  cc.  of  H2SO4  with  20  drops  of  C2H5OH. 

2.  The  presence  of  magnesium  or  of  strontium  in  the  K2C2O4  pre- 
cipitate does  not  interfere  with  the  test.  For,  even  though  a  moderate 
quantity  of  magnesium  passed  into  the  H2SO4  solution,  it  would  not 
precipitate  on  the  addition  of  C2H5OH ;  and  the  quantity  of  strontium 
which  dissolves  in  the  H2SO4  mixture  gives  a  barely  noticeable  turbidity 
when  the  two  volumes  of  CaHjOH  are  added.  A  considerable  turbidity 


120  ANALYSIS  OF  ALKALINE-EARTH   GROUP  P.  79 

would  result  if  the  amount  of  strontium  dissolved  by  the  H2SO4  were 
not  reduced  by  mixing  with  it  the  20  drops  of  C2H5OH. 

Procedure  78.  —  Detection  of  Magnesium.  —  To  the  filtrate 
from  the  K2C204  precipitate  (P.  76)  add  5  cc.  of  15  n.  NH4OH 
and  25  cc.  of  Na2HPO4  solution ;  cool,  and  shake  the  mixture ; 
if  no  precipitate  forms,  let  the  mixture  stand  for  at  least  half 
an  hour,  shaking  it  frequently.  (White  precipitate,  presence 
of  MAGNESIUM.)  Filter  out  the  precipitate,  wash  it  once  with 
C2H5OH,  and  treat  it  by  P.  79. 

Notes.  —  i.  Mg(NH4)P04  is  fairly  soluble  even  in  cold  water,  owing 
chiefly  to  hydrolysis  into  NKtOH  and  Mg++HPO4=.  To  diminish  this 
hydrolysis  the  solution  is  made  strongly  ammoniacal. 

2.  In  an  aqueous  solution  this  substance  shows  a  great  tendency  to 
form  a  supersaturated  solution,  and  it  is  therefore  usually  directed  to 
make  the  test  in  as  small  a  volume  as  possible.  In  the  presence  of 
QjHjOH,  however,  precipitation  takes  place  rapidly,  and  even  \  mg. 
of  magnesium  produces  a  distinct  turbidity  within  half  an  hour  under 
the  conditions  of  the  Procedure.  A  small  precipitate  of  this  kind  settles 
out  on  further  standing,  and  may  then  be  detected  by  rotating  the 
solution  so  as  to  cause  the  precipitate  to  collect  in  the  center. 

Procedure  79. — Confirmatory  Test  for  Magnesium. — Treat  the 
Na2HP04  precipitate  (P.  78),  or  a  small  portion  of  it  if  it  is  large, 
with  5  cc.  of  2  n.  H2SO4 ;  add  to  the  solution  10  cc.  of  C2H5OH, 
and  shake  it  continuously  for  two  or  three  minutes.  Filter,  if 
there  is  a  precipitate ;  add  to  the  filtrate  10  cc.  of  water,  20  cc. 
of  NH4OH,  and  5  cc.  of  Na2HPO4  solution ;  and  let  the  mixture 
stand  at  least  half  an  hour.  (White  crystalline  precipitate, 
presence  of  MAGNESIUM.) 

Notes.  —  i.  This  confirmatory  test  should  be  tried  whenever 
NazHP04  produces  a  small  precipitate  that  is  not  distinctly  crystalline. 
For,  if  even  a  small  quantity  of  strontium  or  calcium  failed  to  be  pre- 
cipitated in  P.  74  or  76,  a  flocculent  precipitate  of  Sr3(P04)2  or  Ca3(PO4)2 
would  come  down  on  the  addition  of  NazHPO4. 

2.  The  addition  of  H2S04  and  C2H5OH  precipitates  strontium  and 
calcium  so  completely  that  any  precipitate  (more  than  a  very  slight 
turbidity)  produced  on  adding  Na2HPO4  to  the  H2S04  solution  cannot 
be  due  to  these  elements. 


P.  81 


ANALYSIS  OF  ALKALI-GROUP 


121 


ANALYSIS  OF  THE  ALKALI-GROUP 


GENERAL   DISCUSSION 

Two  methods  of  analysis  of  the  alkali-group  are  here  pre- 
sented. In  one  of  these,  which  is  called  the  "  shorter  less  exact 
method,"  the  two  elements,  potassium  and  sodium,  are  not 
separated,  but  are  tested  for  in  different  portions  of  the  solution. 
In  the  other  method,  called  the  "  exact  method,"  these  ele- 
ments are  separated  by  the  perchloric-acid  process  commonly 
employed  in  quantitative  analysis.  As  its  name  implies,  the 
first  method  is  simpler  and  more  rapidly  executed;  but  it  in- 
volves a  less  delicate,  quantitative,  and  reliable  detection  of 
sodium  than  does  the  second  method.  From  a  pedagogic  stand- 
point the  former  method  will  naturally  be  preferred,  on  account 
of  its  simplicity,  in  brief  courses  of  instruction;  and  the  latter 
method,  in  more  thorough  courses.  From  a  practical  analytical 
standpoint  the  former  method  is  employed  with  advantage  where 
the  detection  of  small  quantities  of  sodium  is  not  important; 
but  the  latter  method  must  be  adopted  in  a  complete  analysis 
where  a  satisfactory  detection  of  sodium  and  an  approximate 
estimate  of  its  quantity  is  desired. 


SHORTER  LESS  EXACT  METHOD 

TABLE  XII.  —  ANALYSIS  OF  THE  ALKALI-GROUP. 

Filtrate  from  the  Ammonium  Carbonate  Precipitate :  NH4,  K,  Na  salts. 
Evaporate,  ignite,  add  HCl,  ignite  again  (P.  81). 


Vapor: 
NH4  salts. 

Residue  :  KC1,  NaCl. 
Add  3  cc.  of  water,  and  treat  portions  as  follows  : 

Add  Na3Co(NO2)6  (P.  82). 

Add  KtHiSbiOj  (P.  83). 

Yellow  precipitate  : 
K2NaCo(NO2)6. 
Test  in  flame. 

Crystalline  precipitate  : 
Na2H2Sb2O7. 

Violet  color  :  K. 

122  ANALYSIS  OF  ALKALI-GROUP  P.  81 

Procedure  81.  —  Removal  of  Ammonium  Salts.  —  Evaporate 
the  filtrate  from  the  (NH^COs  precipitate  (P.  71)  to  dryness  in 
a  small  casserole,  and  ignite  the  residue,  at  first  moderately, 
then  to  a  temperature  much  below  redness,  till  no  more  white 
fumes  come  off,  keeping  the  dish  in  motion  over  the  flame,  and 
taking  care  to  heat  the  sides  as  well  as  the  bottom  of  the  dish. 
Cool  the  dish,  and  pour  into  it  5  cc.  of  12  n.  HCL  (In  case  there 
is  a  considerable  residue,  heat  the  mixture  to  70-80°,  stirring 
with  a  glass  rod  to  disintegrate  the  residue ;  then  cool  the  mixture 
completely,  and  decant  the  solution  from  any  crystalline  residue 
into  another  casserole;  adding  2  cc.  more  12  n.  HC1,  and  de- 
canting again  into  the  same  casserole,  if  the  residue  is  large  enough 
to  retain  much  of  the  solution.)  Evaporate  the  solution  to  dry- 
ness,  and  ignite  the  residue  as  before.  Cool  the  dish,  add  3  cc. 
of  water,  amV  pour  the  solution  through  a  very  small  filter. 
Treat  one-third  of  the  solution  by  P.  82  (to  detect  potassium), 
and  the  remaining  two-thirds  by  P.  83  (to  detect  sodium). 

Notes.  —  i.  Great  care  must  be  taken  to  volatilize  the  ammonium 
salts  completely,  since  even  i  mg.  of  ammonium  would  give  a  pre- 
cipitate in  the  subsequent  test  for  potassium.  To  insure  their  removal 
the  residue  is  ignited  twice.  The  dish  must  not,  however,  be  heated 
nearly  to  a  red  heat  during  the  ignition,  since  at  that  temperature  KC1 
and  NaCl  are  somewhat  volatile. 

2.  Since  in  the  dry  form  even  a  residue  that  seems  very  small  may 
correspond  to  an  appreciable  quantity  of  potassium  or  sodium,  the 
subsequent  tests  for  these  elements  should  be  made  if  there  is  any  residue 
whatever  after  the  final  ignition. 

3.  A  brown  or  black  residue  of  organic  matter,  coming  from  im- 
purity hi  the  ammonium  salts  added  in  the  course  of  analysis  and  from 
the  alcohol  and  filter  paper,  may  remain  upon  treating  the  ignited 
residue  with  water.    There  may  also  be  a  white  residue  of  silica,  com- 
ing from  the  action  of  the  reagents  on  the  glass  and  porcelain  vessels 
throughout  the  course  of  the  analysis. 

4.  The  addition  of  the  5  cc.  of  12  n.  HC1  serves  to  leave  undissolved 
as  KC1  all  but  about  50  mg.  of  the  potassium  when  a  larger  quantity 
of  it  is  present.    This  is  important  since  it  makes  the  subsequent  sodium 
test  much  more  delicate.     When  much  sodium  is  present  a  large  quan- 
tity of  it  is  also  left  undissolved.    The  ignited  residue  is  warmed  with 
the  HC1,  so  as  to  dissolve  it  partially ;  since  treatment  with  the  acid 


P.  82  ANALYSIS  OF  ALKALI-GROUP  123 

in  the  cold  may  extract  scarcely  any  of  a  small  quantity  of  either  of 
the  alkali-group  elements  from  a  large  residue  of  the  other. 

5.  Only  3  cc.  of  water  are  added  to  the  residue  after  the  second 
ignition  so  that  the  volume  may  be  small  enough  to  enable  the  subse- 
quent test  for  sodium  to  be  applied  directly,  without  evaporating  the 
solution. 

Procedure  82.  —  Detection  of  Potassium.  —  Dilute  one- third 
of  the  solution  (P.  81)  to  5  cc.,  and  add  an  equal  volume  of 
Na3Co(N02)6  reagent.  If  no  precipitate  forms  at  once,  let  the 
mixture  stand  for  at  least  10  minutes.  Filter,  and  wash  the 
precipitate  thoroughly  with  water.  (Yellow  precipitate,  presence 
of  POTASSIUM.) 

Treat  the  precipitate  with  a  5-cc.  portion  of  hot  HC1,  evaporate 
the  solution  to  1-2  drops,  dip  into  it  a  clean  platinum  wire 
(which  has  been  heated  in  a  flame  till  it  no  longer  colors  it), 
and  introduce  the  wire  into  a  colorless  gas  flame,  viewing  the 
flame  through  a  sufficient  thickness  of  blue  cobalt  glass  to  cut 
off  sodium  light.  (Violet  flame,  presence  of  POTASSIUM.) 

Notes.  —  i.  The  Na3Co(NO2)e  (sodium  cobaltinitrite)  reagent  is  a 
solution  o.i  formal  in  Na3Co(NO2)6,  3  n.  in  NaN02,  and  i  n.  in  HAc. 

2.  The  presence  of  0.3  mg.  of  potassium  in  5  cc.  of  solution  may  be 
detected  within  5  or  10  minutes,  and  an  even  smaller  amount  on  long 
standing.    The  yellow  color  of  the  precipitate  is  best  seen  on  the  filter 
after  washing  out  the  NasCo(NO2)6  thoroughly. 

3.  Even  0.5-1.0  mg.  of  ammonium  produces  a  precipitate  very 
similar  in  appearance  to  that  given  by  potassium.     Moderate  amounts 
of  the  alkaline-earth  elements  do  not  interfere  with  the  test. 

4.  The  flame  coloration  produced  by  sodium  is  so  much  more  deli- 
cate than  that  caused  by  potassium  that  the  presence  of  a  minute  quan- 
tity of  sodium  may  completely  obscure  the  color  given  by  a  moderate 
amount  of  potassium.  A  sufficient  thickness  of  blue  cobalt  glass  is 
therefore  used  to  absorb  the  yellow  rays  completely,  and  thus  permit 
the  violet  rays  due  to  potassium  to  be  seen.  It  is  necessary  to  use  two 
or  three  pieces  of  the  blue  glass  usually  supplied  for  the  purpose.  Com- 
parative experiments  with  known  solutions  ought  always  to  be  made, 
unless  the  analyst  is  familiar  with  the  appearance  of  the  flames. 

5.  This  confirmatory  flame  test  should  not  be  omitted  (unless  the 
yellow  precipitate  is  large),  owing  to  the  danger  of  error  arising  from 
incomplete  removal  of  the  ammonium  salts  in  the  ignitions  in  P.  81. 


124  ANALYSIS  OF  ALKALI-GROUP  P.  83 

Procedure  83.  —  Detection  of  Sodium.  —  To  the  remaining 
two-thirds  of  the  solution  (P.  81)  add  2  cc.  of  K2H2Sb207  reagent  ; 
pour  the  mixture  into  a  test-tube,  and  let  it  stand  for  at  least 
half  an  hour,  or  better  overnight.  In  case  there  is  a  flocculent 
precipitate,  shake  the  mixture,  and  after  a  few  seconds  decant 
the  liquid  and  the  suspended  precipitate.  (White  crystalline 
precipitate,  presence  of  SODIUM.) 


Notes.  —  i.  The  K^H^SbzOr  (dispotassium  dihydrogen  pyroanti- 
monate)  reagent  is  0.05  formal  in  this  salt  and  o.i  n.  in  KOH.  The 
alkali  must  be  present,  since  the  pyroantimonate  decomposes  rapidly 
in  acid  solution,  and  slowly  in  neutral  solution,  with  precipitation  of 
HSbOs  (metantimonic  acid).  The  reagent  should  be  tested  occasion- 
ally with  a  solution  containing  i  or  2  mg.  of  sodium,  to  make  sure  that 
it  is  in  good  condition. 

2.  The  precipitate  of  NajHzSbjOr  is  a  heavy  crystalline  one,  which 
usually  adheres  in  part  to  the  glass,  where  it  can  best  be  seen  by  tilting 
the  test-tube  or  pouring  the  liquid  out  of  it.     A  flocculent  precipitate 
affords  no  evidence  of  the  presence  of  sodium. 

3.  When  the  solution  contains  no  other  basic  constituent  i  mg.  of 
sodium  is  easily  detected.    In  the  presence  of  much  potassium  the 
test  is  far  less  delicate.    Thus  3  to  10  mg.  of  sodium  would  have  to  be 
present  to  give  a  precipitate  if  the  2  cc.  of  solution  contained  more 
than  loomg.  of  potassium.    The  treatment  with  HC1  in  P.  81,  however, 
so  decreases  the  quantity  of  potassium  that  can  be  present  that  2  or 
3  mg.  of  sodium  can  be  detected. 

4.  With   the   KzHzSbzO?   reagent  many   other  elements,   even  if 
present  in  small  quantity,  give  precipitates.     Thus  a  distinct  turbidity 
is  produced  by  even  0.1-0.2  mg.  of  alkaline-earth  elements  —  quanti- 
ties which  might  not  have  been  removed  by  (NH^COs  in  P.  71.    These 
elements  yield,  however,  light,  flocculent  precipitates  which  look  very 
different  from  the  heavy  crystalline  precipitate  obtained  with  sodium, 
especially  if  the  mixture  has  been  allowed  to  stand  a  few  hours.     Even 
though,  as  is  often  the  case,  such  a  flocculent  precipitate  is  produced, 
the  presence  of  a  crystalline  precipitate  can  usually  be  detected  by 
decanting  the  liquid  as  directed  in  the  Procedure. 


P.  85 


ANALYSIS  OF  ALKALI-GROUP 


125 


ANALYSIS   OF   THE   ALKALI   GROUP 
EXACT  METHOD 


TABLE  XIII.  —  ANALYSIS  OF  THE  ALKALI-GROUP. 

Filtrate  from  the  Ammonium  Carbonate  Precipitate :  NH4,  K,  Na  salts. 

Evaporate,  and  ignite  the  residue.  Dissolve  in  water,  add  BaClz  (to 
remove  sulfate),  then  (NH^COs  (to  remove  barium).  Evaporate 
and  ignite  again  (P.  8$). 


Vapor  : 
NH4  salts. 

Residue  :  KC1,  NaCl.   Add  HCIO*,  evaporate,  add  alcohol  (P.  86}  . 

Residue:  KC1O4. 
Dissolve  in  hot  water, 
add  Na£o(NOt)t 
(P.  8?}. 

Solution:  NaClO4. 
Saturate  with  HCl  gas  (P.  88). 

Precipitate:  NaCl. 
Dissolve  in  water,  add 
KJEL^Sb^)-,  (P.  89). 

Filtrate  : 
Reject. 

Yellow  precipitate  : 
K2NaCo(N02)6. 

Crystalline  precipitate  : 
Na2H2Sb207. 

Procedure  85.  —  Removal  of  Sulfate  and  of  Ammonium  Salts. 
—  Evaporate  the  filtrate  from  the  (NH4)2CO3  precipitate  (P.  71) 
to  dryness  in  a  small  casserole,  and  ignite  the  residue,  at  first 
very  moderately,  then  to  a  temperature  much  below  redness, 
till  no  more  white  fumes  come  off,  keeping  the  dish  in  motion 
over  a  flame  and  taking  care  to  heat  the  sides  as  well  as  the 
bottom  of  the  dish.  Cool  the  dish,  add  5  cc.  of  water,  transfer 
the  solution  to  a  flask,  and  add  2-10  cc.  of  BaCl2  solution.  (In 
case  H2S04  was  used  in  P.  5  or  P.  8  in  the  Preparation  of  the 
Solution,  add  enough  more  BaCl2  solution  to  precipitate  all  the 
sulfate.)  Heat  the  mixture  nearly  to  boiling  for  2  or  3  minutes, 
and  filter  out  the  precipitate.  To  the  nitrate  add  5-15  cc.  of 
(NH4)2C03  reagent,  let  the  mixture  stand  for  5  minutes,  heat  it 
nearly  to  boiling,  and  filter  out  the  precipitate.  Evaporate  the 
filtrate  to  dryness  in  a  small  casserole,  and  ignite  the  residue 
just  as  before.  Cool  the  dish,  add  5  cc.  of  water,  filter  the 
mixture  through  a  5-cm.  filter,  evaporate  the  filtrate  to  dryness 


126  ANALYSIS  OF  ALKALI -GROUP  P.  86 

in  a  small  casserole,  and  ignite  the  residue  as  before.  (White 
residue,  presence  of  POTASSIUM  or  SODIUM.)  Treat  the  residue 
by  P.  86. 

Notes.  —  i.  Sulfate  must  be  removed  before  attempting  to  separate 
potassium  and  sodium  with  HC1O4.  The  process  of  removing  it  in- 
volves its  precipitation  as  BaSO4  by  the  addition  of  BaCl2,  the  removal 
of  the  excess  of  barium  by  the  subsequent  addition  of  (NH4)2C03, 
and  finally  the  volatilization  of  the  ammonium  salts  by  ignition.  If 
phosphate  is  present,  it  is  also  removed  by  the  addition  of  BaCl2  to 
the  neutral  solution ;  but  its  removal  is  not  essential  for  the  HC104 
separation. 

2.  Sulfate  may  be  present  in  the  (NH4)2CO3  filtrate  in  considerable 
quantity  either  because  sulfate  (or  sulfide)  is  a  constituent  of  the  sub- 
stance submitted  to  analysis,  or  because  H2SO4  was  used  hi  the  prep- 

'  aration  of  the  solution  in  P.  5  or  P.  8.  Even  when  it  does  not  come 
from  these  sources,  a  small  quantity  of  it  is  usually  present  at  this 
point  in  the  analysis,  because  it  is  produced  when  the  (NH4)2CO3 
filtrate  is  evaporated  to  dryness  and  the  residue  is  ignited,  owing  to 
the  action  of  the  nitrate  present  on  sulfur-compounds  coming  from 
the  decomposition  of  the  (NH4)2S  reagent.  The  removal  of  sulfate  is 
therefore  made  a  part  of  the  regular  procedure. 

3.  As  to  the  precautions  to  be  observed  in  igniting  the  residue  and 
as  to  the  residue  itself,  see  Notes  1-3,  P.  8 1. 

Procedure  86.  —  Separation  of  Potassium  and  Sodium.  —  To 
the  ignited  residue  (P.  85)  add  2-5  cc.  of  6  n.  HC104;  and 
evaporate,  by  keeping  the  dish  in  motion  over  a  small  flame,  till 
thick  white  fumes  of  HC1O4  come  off  copiously.  Cool  completely, 
add  10-20  cc.  of  95%  C2H5OH,  and  stir  the  mixture  for  2-5 
minutes  if  there  is  much  residue.  (White  residue,  presence  of 
POTASSIUM.)  Filter  through  a  dry  filter-paper,  and  wash  the 
residue  with  a  little  95%  C2H5OH.  (Precipitate,  P.  87  ;  filtrate, 
P.  88.) 

Notes.  —  i.  Enough  HC1O4  must  be  added  to  convert  the  potassium 
and  sodium  chlorides  completely  into  per  chlorates,  and  the  evaporation 
must  be  continued  till  all  the  HC1  is  expelled;  for  otherwise  Nad, 
being  insoluble  in  alcohol,  may  be  left  as  a  residue  with  the  KC1O4. 
An  unnecessary  excess  of  HC104  is,  however,  to  be  avoided,  since  it 
makes  the  subsequent  test  for  sodium  somewhat  less  delicate.  The 


P.  88  ANALYSIS  OF  ALKALI-GROUP  127 

quantity  added  is  therefore  varied  (from  2  to  5  cc.)  in  accordance  with 
the  size  of  the  ignited  residue  obtained  in  P.  85. 

2.  Another  reason  for  continuing  the  evaporation  till  the  HC1O4 
fumes  strongly  is  to  remove  most  of  the  water ;  for  this  test  for  potas- 
sium and  the  subsequent  test  for  sodium  (in  P.  88)  are  more  delicate, 
the  less  the  quantity  of  water  present.    When  the  directions  given  in 
the  Procedure  are  followed,  i-ij  mg.  of  potassium  produces  a  distinct 
precipitate. 

3.  If  sulfate  were  present  and  it  were  not  removed  in  P.  85  by  the 
addition  of  BaCl2,  this  separation  of  potassium  and  sodium  would  be 
unsatisfactory;  for  Na2SO4  would  remain  in  the  residue  undissolved 
by  the  alcohol.    This  arises  from  the  fact  that  H2SO4  is  less  volatile 
than  HC1O4  and  is  therefore  not  expelled  by  it  in  the  evaporation,  and 
from  the  fact  that  Na2SO4  is  only  slightly  soluble  in  alcohol  even  in 
the  presence  of  HC1O4. 

4.  The  presence  of  phosphate  or  borate  does  not  interfere  with  the 
separation ;  for,  though  phosphoric  and  boric  acids  are  not  volatilized 
in  the  evaporation  with  HC1O4,  they  are  displaced  from  their  sodium 
salts  by  the  excess  of  HC104,  since  this  acid  is  much  more  largely  ionized 
than  phosphoric  or  boric  acid.    The  sodium  therefore  remains  dissolved 
in  the  alcoholic  solution. 

Procedure  87.  —  Confirmatory  Test  for  Potassium.  —  Pour  re- 
peatedly through  the  filter  containing  the  HC1O4  precipitate 
(P.  86)  a  5-10  cc.  portion  of  boiling  water.  Cool  the  mixture, 
add  to  it  5  cc.  of  Na3Co(NO2)e  reagent,  and  let  it  stand  for  10 
minutes.  (Yellow  precipitate,  presence  of  POTASSIUM.) 

Notes.  —  i.  This  confirmatory  test  should  not  be  omitted,  owing  to 
the  possibility  that  the  residue  left  undissolved  by  the  alcohol  in  P.  86 
consists  of  NaClO4,  NaCl,  or  Na2SO4.  The  NaClO4  may  result  from 
incomplete  solution  of  it  in  the  C2HsOH,  the  NaCl  from  incomplete 
conversion  of  the  chlorides  into  perchlorates  in  the  evaporation  with 
HC1O4,  and  the  Na2S04  from  the  presence  of  sulfate  which  was  not 
removed  by  the  BaCl2. 

2.   In  regard  to  this  test  for  potassium  see  Notes  1-3,  P.  82. 

Procedure  88.  —  Detection  of  Sodium.  —  Pour  the  alcoholic 
filtrate  (P.  86)  into  a  dry  conical  flask  placed  in  a  vessel  of  cold 
water,  and  pass  into  it  a  fairly  rapid  current  of  dry  HC1  gas 
(see  Note  2)  till  the  gas  is  no  longer  absorbed.  (White  pre- 


128  ANALYSIS  OF  ALKALI-GROUP  P.  89 

cipitate,  presence  of  SODIUM.)  Filter  through  a  small  filter- 
paper.  Wash  the  precipitate  with  a  little  99  %  C2H5OH,  and 
treat  it  by  P.  89.  Reject  the  nitrate :  do  not  under  any  circum- 
stances heat  or  evaporate  it. 

Notes.  —  i.  This  test  for  sodium  is  most  delicate  when  the  alcohol 
is  completely  saturated  with  HC1  gas,  in  which  case  i  mg.  of  sodium 
can  be  detected. 

2.  The  dry  HC1  gas  may  be  prepared  by  dropping  95%  H2SO4 
from  a  separating  funnel  into  a  flask  containing  solid  NaCl  covered 
with  12  n.  HC1,  and  passing  the  gas  through  a  gas- wash-bottle  con- 
taming  95%  H2SO4.     Such  a  gas-generator  may  be  conveniently  kept 
ready  for  use  in  a  hood  in  the  laboratory,  as  the  evolution  of  gas  soon 
ceases  when  no  more  H2S04  is  added. 

3.  The  alcoholic  filtrate  containing  the  excess  of  HC104  must  not 
be  heated  or  evaporated,  since  a  dangerous  explosion  is  likely  to  result. 

Procedure  89.  —  Confirmatory  Dry  Test  for  Sodium.  —  Pour 
a  lo-cc.  portion  of  water  repeatedly  through  the  filter  containing 
the  HC1  precipitate  (P.  88),  evaporate  the  solution  just  to  dry- 
ness,  add  i  cc.  of  water,  then  KOH  solution  drop  by  drop  till  the 
mixture  turns  litmus  paper  blue,  and  finally  2  cc.  of  K2H2Sb2O7 
reagent.  Pour  the  mixture  into  a  test-tube,  and  let  it  stand  for 
at  least  half  an  hour,  preferably  overnight.  In  case  there  is  a 
flocculent  precipitate,  shake  the  mixture,  and  after  a  few  seconds 
decant  the  liquid  and  the  suspended  precipitate.  (White  crystal- 
line precipitate,  presence  of  SODIUM.) 

Notes.  —  i.  In  regard  to  this  test  for  sodium  see  the  Notes  on 
P.  83. 

2.  Since  there  is  no  potassium  present,  this  test  for  sodium  is  much 
more  delicate  than  the  corresponding  one  made  in  the  shorter  method 
of  analysis  of  the  alkali-group. 


SUPPLEMENTARY  PROCEDURES  129 

SUPPLEMENTARY   PROCEDURES   FOR   BASIC   CONSTITUENTS 


GENERAL  DISCUSSION 

The  system  of  analysis  for  the  basic  constituents  presented 
in  the  preceding  Procedures  needs  to  be  supplemented  by  pro- 
vision for  the  detection  of  ammonium  and  for  the  determination 
of  the  state  of  oxidation  of  certain  elements. 

The  detection  of  ammonium  is  provided  for  in  P.  91.  Since 
ammonium  does  not  occur  in  substances  of  natural  origin  nor 
in  industrial  products  that  have  been  made  by  high-temperature 
processes,  this  Procedure  is  omitted  in  analyzing  such  substances. 

The  elements  forming  basic  constituents  which  commonly 
occur  in  two  or  more  states  of  oxidation  are  mercury,  iron,  tin, 
arsenic,  chromium,  and  manganese.  A  method  is  presented 
in  P.  92  for  determining  the  state  of  oxidation  of  the  first  three 
of  these  elements,  which  form  the  cations  Hg2++  and  Hg++,  Fe++ 
and  Fe+++,  and  Sn++  and  Sn++-|-f  (also  in  the  case  of  tin  the 
corresponding  anions  Sn02™  and  SnOa"5).  Arsenic  is  ordinarily 
in  the  form  of  the  acidic  constituents,  arsenite  and  arsenate, 
forming  the  anions  AsO2~  and  As04-  (in  which  the  arsenic  has 
the  valence  3  and  5,  respectively)  ;  and  these  are  provided  for 
(in  P.  1 1 6)  in  the  system  for  the  detection  of  acidic  constituents. 
Chromium  is  commonly  met  with,  either  as  a  basic  constituent 
in  chromic  compounds,  forming  the  cation  Cr+++,  or  as  an  acid 
constituent  in  chromates,  forming  the  anions  CrO4=  and  Cr2Ov" 
(in  which  the  chromium  has  a  valence  of  6).  Whether  or  not 
it  is  present  as  chromate  is  determined  in  P.  in.  Manganese 
commonly  occurs  as  a  basic  constituent  in  manganous  compounds, 
forming  the  cation  Mn++,  as  the  oxide  Mn02  (in  which  the  man- 
ganese has  the  valence  4),  or  as  an  acidic  constituent  in  man- 
ganates  and  permanganates,  forming  the  anions  Mn04=  and 
MnO4~~  (in  which  the  manganese  has  the  valence  6  and  7,  re- 
spectively). The  green  and  purple  colors  of  the  last  two  of 
these  constituents  are  so  pronounced  and  undergo  such  marked 
changes  in  the  treatment  with  acids  (in  P.  3)  or  with  H2S  (in  P. 
21)  that  special  provision  is  not  made  for  their  detection. 


130  SUPPLEMENTARY  PROCEDURES  P.  91 

TABLE  XIV.  —  SUPPLEMENTARY  PROCEDURES  FOR  BASIC  CONSTITUENTS 


Boil  the  substance 
with  NaOH  solu- 
tion (P.  91). 

Boil  the  substance  with  H^SO*  ;  treat  portions  of  the  solution  as  follows 
(P.  92.)  : 

Add  K3Fe(CN)t. 

Add  KSCN. 

Add  EgCh. 

Add  HCl. 

Vapor  •  NHa 

Absorb  in 

Blue  precipitate: 
Fe3(Fe(CN)8)2. 

Red  color: 
Fe(SCN)3. 

Precipitate  : 
Hg2Cl2. 

Precipitate  : 

Filtrate. 

(ShoWS  FERROUS 

(Shows 

(Shows 

HgjClj  or 

HgCl2. 

KtHglt. 

IRON.) 

FERRIC 

STANNOUS 

AgCl. 

Add  SnCl.i 

IRON.) 

Add 

Orange  precipitate  : 

NHtOH. 

Precipitate: 

(Shows 
AMMONIUM.) 

Black 
residue  : 
Hgand 

orHg.2 
(Shows 

MERCURIC 

HgNHl  . 

MERCURY.) 

(Shows 

MERCUROUS 

MERCURY.) 

Procedure  91.  —  Detection  of  Ammonium.  —  Place  0.2  g.  of 
the  finely  powdered  substance  and  2  cc.  of  NaOH  solution  in  a 
50-cc.  round-bottom  flask.  Insert  a  stopper  carrying  a  glass 
rod  around  whose  end  is  wound  a  piece  of  moist  red  litmus  paper ; 
and  heat  the  mixture  nearly  to  boiling.  (Blue  coloration  of  the 
litmus  paper  and  odor  of  ammonia,  presence  of  AMMONIUM.) 

Confirmation  of  Ammonium.  —  If  the  litmus  turns  blue  or  the 
vapors  smell  of  ammonia,  pour  into  the  flask  10  cc.  of  water, 
insert  a  stopper  fitted  with  a  long  wide  delivery-tube  leading  to 
the  bottom  of  a  test-tube  placed  in  a  vessel  of  cold  water,  and 
distil  slowly  till  about  half  the  water  has  passed  over.  To  the 
distillate  add  K2HgI4  reagent,  drop  by  drop,  so  long  as  the  pre- 
cipitate increases.  (Orange  precipitate,  presence  of  AMMONIUM.) 

Notes.  —  i.  Less  than  0.2  mg.  of  ammonium  can  be  detected  with 
litmus  paper  and  by  the  odor  when  the  test  is  carried  out  as  described 
in  the  first  paragraph  of  the  Procedure.  The  test  described  in  the  last 
paragraph  is  useful  with  very  small  quantities  of  ammonium  as  a 
confirmation,  and  with  larger  quantities  as  a  means  of  better  estimating 
the  proportion  of  it  present. 

2.  The  K2HgI4  reagent  is  a  solution  0.5  n.  in  KzHgl*  and  3  n.  in 
NaOH.  It  is  commonly  called  Nessler  reagent. 


P.  92  SUPPLEMENTARY  PROCEDURES  131 

3.  The  orange  precipitate  produced  by  the  action  of  NH3  on  alkaline 
KzHg^  is  a  complex  compound  of  the  composition  HgO  •  Hg(NH2)I. 
The  test  is  extremely  delicate,  a  distinct  precipitate  resulting  even 
with  0.2  mg.  of  ammonium  in  5  cc.  of  solution,  and  a  pronounced  yellow 
color  with  a  much  smaller  quantity.  This  fact  must  be  taken  into 
account  in  estimating  the  quantity  of  ammonium  present. 

Procedure  92.  —  Determination  of  the  State  of  Oxidation  of 
Iron,  Tin,  and  Mercury.  —  In  case  iron,  tin,  or  mercury  has 
been  found  present,  boil  20  cc.  of  H2S04  in  a  small  flask ;  drop 
into  it  0.2  g.  of  the  finely  powdered  substance.  (If  solution  does 
not  take  place  at  once,  boil  the  mixture  vigorously  for  2-3 
minutes,  covering  the  flask  loosely  with  a  watch-glass.)  Cork 
the  flask,  cool  the  mixture,  and  treat  immediately  5-cc.  portions 
of  it  as  follows,  first  pouring  them  through  a  filter  if  the  substance 
has  not  dissolved  completely. 

In  case  iron  has  been  found  present,  pour  one  portion  into  3  cc. 
of  K3Fe(CN)6  solution  (blue  precipitate,  presence  of  FERROUS 
IRON)  ;  and  pour  another  portion  into  3  cc.  of  KSCN  solution 
(red  color,  presence  of  FERRIC  IRON). 

In  case  tin  has  been  found  present,  pour  one  portion  into  5  cc. 
of  0.2  n.  HgCl2  solution.  (White  precipitate,  presence  of  STAN- 
NOUS  TIN.) 

In  case  mercury  has  been  found  present,  to  one  portion  add 
2  cc.  of  HC1.  (White  precipitate,  presence  of  MERCUROUS  MER- 
CURY or  SILVER.)  Filter  the  mixture.  Treat  the  precipitate  on 
the  filter  with  NH4OH.  (Black  residue,  presence  of  MERCUROUS 
MERCURY.)  To  the  filtrate  add  SnCl2  solution,  one  drop  at  first, 
then  a  few  more  drops,  and  finally  1-2  cc.  (White  precipitate, 
turning  gray  with  excess  of  the  reagent,  presence  of  MERCURIC 

MERCURY.) 

Notes.  —  i.  The  H2S04  is  boiled  before  adding  the  substance  in 
order  to  expel  the  air,  which  would  oxidize  stannous  and  ferrous  salts. 
The  mixture  is  afterwards  boiled  in  order  to  decompose  slowly  dissolv- 
ing substances,  and  in  order  to  expel  any  H2S  (arising  from  the  presence 
of  a  sulfide),  which  would  give  a  precipitate  with  the  SnCl2  and  HgCl2. 

2.  If  in  preparing  the  solution  for  the  analysis  for  basic  constituents 
the  substance  was  dissolved  in  water  or  in  cold  dilute  HNOs,  the  state 


132  SUPPLEMENTARY  PROCEDURES  P.  92 

of  oxidation  of  mercury  will  have  been  determined  by  its  presence  or 
absence  in  the  HC1  and  H2S  precipitates.  But,  if  the  substance  was 
treated  with  hot  or  concentrated  HNO3,  any  mercurous  compound 
present  will  have  been  partly  or  completely  oxidized  to  the  mercuric 
state. 


DETECTION  OF  THE  ACIDIC  CONSTITUENTS 


GENERAL    DISCUSSION 

THE  acidic  constituents  whose  detection  is  here  provided  for  are : 

Arsenate  Chromate  Nitrate 

Arsenite  Cyanide  Nitrite 

Borate  Ferrocyanide  Oxalate 

Bromide  Ferricyanide  Sulfate 

Carbonate  Fluoride  Sulfide 

Chlorate  Hypochlorite  Sulfite 

Chloride  Iodide  Thiocyanate 

Provision  has  already  been  made  for  detecting  phosphate  and 
silicate  in  the  course  of  the  analysis  for  basic  constituents. 

Different  processes  are  described  in  this  book  for  the  detection 
of  the  acidic  constituents,  according  as  the  substance  is,  on  the 
one  hand,  an  industrial  product  that  has  been  made  by  a  high- 
temperature  process  or  a  substance  that  is  of  natural  origin; 
or,  on  the  other  hand,  an  industrial  product  that  has  been 
separated  from  solutions  or  prepared  by  other  low-temperature 
process.  The  first  of  these  classes  of  substances  will  be  called 
natural  substances  and  igneous  products;  the  second,  non-igneous 
products.  The  first  class  includes  all  minerals,  ores,  and  rocks 
(except  water-soluble  salt-deposits) ;  slags,  mattes,  and  other 
metallurgical  products;  and  glasses,  porcelains,  refractories, 
abrasives,  and  other  ceramic  products.  The  second  class  includes 
all  other  industrial  products,  such  as  chemicals,  pigments, 
fertilizers,  and  commercial  preparations. 

The  main  reason  for  this  differentiation  is  that  water-insoluble 
minerals  and  high-temperature  products  contain  only  a  com- 
paratively small  number  of  acidic  constituents;  namely,  car- 
bonate, sulfide,  sulfate,  chloride,  fluoride,  borate,  phosphate, 
and  silicate,  and  rarely  cyanide;  so  that  a  much  simpler  pro- 
cedure can  be  followed  than  when  any  acidic  constituent  what- 

133 


134  DISCUSSION  OF  ACIDIC  CONSTITUENTS 

ever  may  be  present.  A  second  reason  is  that  these  natural 
and  igneous  substances  are,  as  a  rule,  decomposed  with  more 
difficulty  than  most  industrial  products,  making  necessary  the 
use  of  strong  acids  in  place  of  the  treatment  with  sodium  car- 
bonate solution,  which  is  best  employed  in  preparing  a  solution 
for  testing  for  acidic  constituents  in  the  more  reactive  substances. 

In  the  following  system  of  analysis  non-igneous  products  are 
first  considered,  since  a  survey  of  the  methods  of  detection  of 
all  the  acidic  constituents  is  thereby  obtained.  (See  Tables 
XV-XX.)  The  much  shorter  process  required  for  the  detection 
of.  the  small  number  of  constituents  that  may  occur  in  natural 
substances  or  igneous  products  is  then  presented.  (See  Tables 
XXI  and  XXII.) 

It  is  to  be  noted  that  the  system  of  procedure  for  detecting 
the  acidic  constituents  can  often  be  much  shortened  by  omitting 
the  tests  for  certain  constituents  which  are  excluded  by  the 
solubility  of  the  substance  considered  in  connection  with  the 
basic  constituents  present.  Thus,  in  a  neutral  water-soluble 
substance  containing  barium  or  silver  it  is  unnecessary  to  test 
for  any  of  the  acidic  constituents  which  form  insoluble  com- 
pounds with  these  elements.  A  General  Statement  as  to  the 
Solubilities  of  substances  in  water  and  dilute  acids  will  be  found 
in  the  Appendix. 

Metals  and  alloys  do  not  contain  any  of  the  ordinary  acidic 
constituents,  but  they  may  contain  the  elements  carbon,  phos- 
phorus, and  silicon  in  considerable  proportion.  These  are 
commonly  detected  in  the  course  of  the  analysis  for  basic 
constituents. 


P.  100        DIRECTIONS  FOR  ACIDIC  CONSTITUENTS  135 

GENERAL  DIRECTIONS 

Procedure  100.  — General  Directions.  —  In  case  the  substance 
is  a  non-igneous  product,  proceed  as  follows : 

Treat  a  sample  of  the  substance  by  P.  101,  to  prepare  a  solu- 
tion for  the  detection  (by  P.  102-116)  of  most  of  the  acidic 
constituents. 

Treat  a  sample  of  the  substance  by  P.  117,  heating  it  with 
dilute  HC1  and  Zn,  and  testing  the  distillate  for  carbonate  and 
sulfide. 

In  case  the  substance  is  a  natural  substance  or  igneous  product, 
proceed  as  follows : 

Treat  a  sample  of  the  substance  by  P.  121,  distilling  it  with 
dilute  HC1  and  Zn ;  testing  the  mixture  remaining  in  the  flask 
for  sulfate,  and  the  distillate  for  carbonate,  sulfide,  and  cyanide. 

Treat  a  sample  of  the  substance  by  P.  122,  distilling  it  with 
H2S04,  first  alone  and  then  with  methyl  alcohol,  to  detect 
chloride,  fluoride,  and  borate. 

If  the  substance  after  treatment  with  HN03  and  HC1  in  P.  2 
and  3  left  a  residue  that  has  been  fused  with  Na2CO3  (by  P.  7), 
treat  the  aqueous  extract  of  the  fused  mass  as  described  in 
P.  123,  to  detect  sulfate,  fluoride,  borate,  and  silicate. 

In  case  the  substance  is  a  solution,  treat  it  as  described  in  the 
last  two  paragraphs  of  P.  9. 

Notes.  —  i.  The  reasons  for  adopting  distinct  systems  of  pro- 
cedures for  "  non-igneous  products  "  and  for  "  natural  substances  and 
igneous  products,"  and  the  significance  of  these  terms  as  here  used, 
have  been  explained  in  the  General  Discussion  on  the  preceding  pages. 

2.  The  much  smaller  number  of  constituents  that  need  to  be  tested 
for  in  the  latter  class  of  substances  has  been  there  referred  to.  They 
are  the  seven  whose  detection  is  provided  for  in  the  second  section  of 
this  Procedure;  and  in  addition,  silicate,  phosphate,  and  arsenate, 
which  are  ordinarily  detected  in  the  analysis  for  basic  constituents. 


ANALYSIS  OF  NON-IGNEOUS  PRODUCTS 


PREPARATION   OF  THE   SOLUTION  AND  DIRECTIONS   FOR  ITS 
TREATMENT 

Procedure  101.  —  Preparation  of  a  Solution  by  Boiling  the 
Substance  with  Sodium  Carbonate  Solution.  —  Place  in  a  casserole 
2\  g.  of  the  finely  powdered  substance  and  25  cc.  of  3  n.  NaaCOs 
solution ;  cover  the  casserole,  and  boil  the  mixture  very  gently 
for  5-10  minutes,  replacing  the  water  if  much  evaporates.  (See 
Notes  3  and  4.)  Filter.  Wash  the  residue,  and  reserve  it  for 
treatment  by  P.  117  in  certain  cases.  Dilute  the  solution  to 
just  30  cc.,  and  treat  portions  of  it  as  follows : 

Treat  i-cc.  portions  of  the  solution  by  P.  102,  104,  and  105, 
and  a  2-cc.  portion  by  P.  103,  to  detect  certain  groups  of  acidic 
constituents. 

In  case  in  P.  102  the  chloride-group  is  found  present,  treat  a 
6-cc.  portion  by  P.  106-110,  to  detect  the  separate  constituents 
of  that  group. 

In  case  in  P.  103  the  sulfate-group  is  found  present,  treat  a 
6-cc.  portion  by  P.  111-112,  to  detect  the  separate  constituents 
of  that  group. 

In  case  in  P.  104  oxidizing  constituents  are  found  present, 
treat  a  2-cc.  portion  by  P.  113,  to  detect  nitrate  or  nitrite;  and, 
if  found  present,  treat  a  i-cc.  portion  by  P.  114,  to  detect  nitrite. 

In  every  case  treat  a  3-cc.  portion  by  P.  115,  to  detect  borate. 

In  case  in  P.  44  arsenic  was  found  present,  treat  a  3-cc.  portion 
by  P.  116,  to  detect  arsenate  and  arsenite. 

Notes.  —  i.  The  treatment  with  Na2CO8  serves  two  purposes.  In 
the  case  of  water-soluble  salts,  it  precipitates  and  removes  from  the 
solution  all  the  basic  constituents,  except  potassium  and  sodium  and 
certain  amphoteric  elements ;  and  in  the  case  of  most  water-insoluble 
compounds,  it  causes  metathesis,  the  acidic  constituent  combining  with 
the  sodium  and  passing  into  the  solution,  and  the  basic  constituent 
combining  with  the  carbonate  and  being  precipitated.  Thus  PbSO< 
is  metathesized  with  formation  of  soluble  NajSO4  and  solid  PbCO3. 
136 


P.  101  PREPARATION  OF  TEE  SOLUTION  137 

2.  The  removal  of  the  basic  constituents  is  desirable,  since  many  of 
them  would  interfere  with  the  tests  for  acidic  constituents  by  pro- 
ducing precipitates  with  the  reagents  added  or  imparting  colors  to  the 
solution.     Thus,  if  an  aqueous  or  HNO3  solution  of  the  substance  were 
tested  directly,  silver  and  mercurous  mercury  would  precipitate  when- 
ever any  chloride  is  added ;  bismuth,  aluminum,  chromium,  and  ferric 
iron  might  separate  when  to  a  HN08  solution  NaAc  is  added;   and 
copper,  nickel,  cobalt,  iron,  and  chromium  would  by  their  own  colors 
interfere  with  the  color  test  for  borate. 

3.  In  case  the  only  basic  constituents  present  are  potassium,  sodium, 
and  ammonium,  an  aqueous  solution  of  the  substance  may  be  used 
for  the  tests  for  the  acidic  constituents,  in  place  of  the  solution  pre- 
pared by  boiling  with  Na»COi  solution. 

4.  In  case  the  substance  is  already  in  aqueous  solution,  to  that 
volume  of  the  solution  which  contains  2.5  g.  of  solid  matter  (as  found 
in  P.  9)  25  cc.  of  3  n.  Na^Os  solution  are  added,  the  mixture  is 
evaporated,  made  up  to  just  30  cc.,  and  filtered,  and  portions  of  the 
filtrate  are  treated  by  P.  102-116. 

5.  Many  sulfides  are  not  decomposed  by  the  Na»CO»  solution. 
Even  the  sulfides  of  elements  of  the  iron  group  are  very  little  acted 
upon.     Further  provision  is  therefore  made  (in  P.  117)  for  the  de- 
tection of  sulfide  by  treating  the  original  substance  or  the  residue  un- 
dissolved  by  Na2COs  solution  with  HC1  and  Zn. 

6.  Of  other  substances  that  may  not  be  attacked  the  following  may 
be  mentioned.     Many  of  the  phosphates  are  but  slightly  acted  upon 
by  Na^COs  solution ;  and  it  is  therefore  advantageous  that  phosphate 
has  already  been  tested  for  in  an  acid  solution  of  the  substance  in  the 
course  of  the  analysis  for  basic  constituents.    BaS04  also  may  be 
only  partially  decomposed;   but  it  is  acted  upon  sufficiently  to  yield 
a  good  test  for  sulfate.     Finally,  the  halides  of  silver  are  not  affected 
at  all  by  the  Na2COa  solution. 

7.  The  principles  involved  in  the  metathesizing  action  of  NajCOi 
solution  are  as  follows.    The  extent  to  which  the  metathesis  of  a 
slightly  soluble  salt  (like  PbSO4)  takes  place  is  determined  by  the  ratio 
of  the  saturation-values  of  the  ion-concentration  product  for  the  salt 
and  for  the  corresponding  carbonate  (PbCO,  in  case  of  PbSO4).    For, 
as  explained  in  Note  6,  P.  n,  in  any  solution  saturated  with  respect  to 
these  substances  the  following  mass-action  expressions,  in  which  SpbSOi 
and  SpbCO,  represent  the  solubilities  of  PbS04  and  PbCOj  in  pure  water 
at  any  given  temperature,  must  both  be  satisfied : 

;  and  (Pb++)  X  (COr)  = 


138  NON-IGNEOUS  PRODUCTS 

Dividing  the  first  of  these  equations  by  the  second,  we  get  the  following  : 
(SOr)/(CO.=)  = 


This  expression  shows  that  PbS04  will  be  metathesized  by 
solution  until  the  concentration  of  sulfate-ion  bears  the  same  ratio  to 
the  concentration  of  carbonate-ion  in  the  solution  as  the  square  of  the 
solubility  of  PbS04  bears  to  the  square  of  the  solubility  of  PbCOt  in 
pure  water.  Since,  as  will  be  seen  by  referring  to  the  Table  of  Solu- 
bilities in  the  Appendix,  this  ratio  for  PbS04  and  PbCO3  has  at  20°  the 
very  large  value  490,000,  it  is  evident  that  PbS04  will  be  completely 
transformed  into  PbCO3  at  20°  by  even  a  small  excess  of  Na2COi 
before  this  equilibrium  ratio  of  (SO4=)  to  (COa^  is  established  in  the 
solution.  But  in  the  case  of  the  salt  BaSO4,  the  ratio  of  the  squares  of 
the  solubilities  has,  as  will  be  seen  from  the  table,  the  small  value  o.oi, 
which  shows  that  the  conversion  of  BaS04  into  BaCOj  will  cease  when 
the  concentration  of  sulfate-ion  becomes  only  i%  of  that  of  carbonate- 
ion.  Still,  by  making  the  latter  concentration  large,  as  we  do  by  using 
a  large  excess  of  Na2CO3  solution,  a  considerable  quantity  even  of 
BaSO4  will  be  metathesized  before  the  conditions  of  equilibrium  are 
established.  —  It  is  to  be  noted  that  the  form  of  the  mass-action  ex- 
pression depends  on  the  valences  of  the  two  ions  of  the  salt  that  is  sub- 
jected to  the  action  of  the  Na2CO3  solution.  Thus  the  expression  is 
different  for  salts  of  the  four  valence  types  exemplified  by  PbSO4, 
PbI2,  Ag2SO4,  and  Agl.  The  corresponding  expressions  can  be  readily 
derived  by  the  method  illustrated  above  in  the  case  of  PbSO4. 

8.  Besides  the  elements  which  form  only  acidic  constituents,  those 
which  form  both  basic  and  acidic  constituents  may  be  present  in  the 
Na2CO3  solution.     Thus,  manganese,  chromium,  and  arsenic  may  be 
present  as  sodium  permanganate,  chromate,  arsenate,  and  arsenite. 
Certain  elements  which  form  amphoteric  hydroxides,  like  aluminum, 
chromium,  antimony,  tin,  and  copper,  may  also  pass  in  small  quantity 
into  the  Na2C03  solution. 

9.  Certain  acidic  constituents  may  be  converted  into  other  con- 
stituents by  boiling  Na2CO3  solution.     Hypochlorite,  when  present 
alone,  decomposes  into  chloride  and  chlorate.     If  it  were  present  with 
a  reducing  acidic  constituent  (namely,  with  sulfide,  sulfite,  or  arsenite), 
or  with  oxidizing  basic  constituents  (namely,  with  lead,  antimony, 
stannous  tin,  ferrous  iron,  nickel,  cobalt,  chromium,  and  manganese), 
it  would  be  converted  into  chloride.     Ferricyanide  is  converted  into 
ferrocyanide,  and  permanganate  into  Mn02,  by  most  of  these  reducing 
substances.    Chromate  is  reduced  by  sulfide  and  arsenite,  and  by  stan- 
nous tin  and  ferrous  iron.    Chlorate,  nitrate,  and  nitrite  are  not  re- 

• 

. 


P.  101  PREPARATION  OF  THE  SOLUTION  139 

duced  in  alkaline  solution  by  other  acidic  constituents.  Sulfide  and 
sulfite  do  not  act  upon  one  another  in  boiling  Na»COi  solution,  though 
they  do  so  instantly  on  acidification. 

10.  It  is  unnecessary,  however,  to  provide  for  detecting  hypochlorite, 
ferricyanide,  permanganate,  or  chromate  when  any  of  the  mentioned 
reducing  substances  that  react  with  them  in  boiling  Na2COj  solution 
is  present ;  for,  on  account  of  their  general  incompatibility,  such  com- 
binations are  not  met  with  in  industrial  products.     Ferricyanide  and 
chromate  will  therefore  be  found  as  such  in  the  Na2COs  solution  in  all 
practical  cases ;   and  hypochlorite  need  be  tested  for  (by  P.  108,  on  a 
fresh  sample  of  the  substance)  only  when  both  chloride  and  chlorate 
are  found  present,  and  when  the  reducing  substances  incompatible 
with  it  are  absent. 

11.  The  constituents,  sulfide,  sulfite,  nitrite,  and  iodide,  which  in 
HC1  or  HaSO<  solution  are  strong  reducing  agents,  are  destroyed  in 
the  boiling  NajCOa  solutions  only  by  the  most  powerful  oxidizing 
agents,  —  hypochlorite,    ferricyanide,    and    permanganate    (and    by 
chromate  in  the  case  of  sulfide).     Sulfite,  however,  is  always  partially 
converted  into  sulf ate  by  the  oxygen  of  the  air. 


140 


BEHAVIOR  TOWARD  GROUP-REAGENTS 


P.  102 


BEHAVIOR   OF   THE   ACIDIC   CONSTITUENTS    TOWARD   GROUP 
REAGENTS 


TABLE  XV.  —  DETECTION  OF  GROUPS  OF  ACIDIC  CONSTITUENTS. 


Sodium  Carbonate  Solution  Containing  All  Acidic  Constituents  (P.  101). 
Treat  portions  as  follows: 


Add  AgN03, 
NaN02,  and  HN03 

(P.  102). 

Add,  HAc, 
BaCli,  and  CaCh 
(P.  103). 

Add  MnCh 
and  HCl 
(P.  104). 

Add  HCl,  FeCh, 
andK3Fe(CN)6 
(P.  105). 

Precipitate  : 
CHLORIDE-GROUP 
(S,CN,Fe(CN)6lv, 
Fe(CN)6m,  SCN, 
Cl,  Br,  I, 
C1O3,  CIO), 
as  Ag  salts. 

Precipitate  : 

SULFATE-GROUP 

(S04,  S03,  Cr04, 
F,C204), 
as  Ba  and  Ca 
salts. 

Dark  Color: 
MnCl3. 

Shows  OXIDIZING 
CONSTITUENTS  : 

Fe(CN)6m,  C103, 
CIO,  CrO4,  N03, 
N02. 

Blue  precipitate  : 
Fe4(Fe(CN)6)3. 

Shows  REDUCING 
CONSTITUENTS  ! 

S,  FeCCN)^,  I, 
SO3,  NO2. 

Procedure  102.  —  Detection  of  the  Chloride-Group.  —  To  i  cc. 
of  the  Na2C03  solution  (P.  101)  add  5  cc.  of  water,  3  drops  of 
(chloride-free)  3  n.  NaNO2  solution,  i  cc.  of  AgNO3  solution, 
and  2  cc.  of  HN03.  (Precipitate,  presence  of  CHLORIDE-GROUP.) 

Notes.  —  i.  In  case  no  precipitate  results,  it  shows  the  absence  of 
all  the  constituents  of  the  chloride-group;  for  the  silver  salts  of  all 
these  (except  those  of  chlorate  and  hypochlorite,  which  on  acidification 
are  reduced  to  chloride  by  the  NaNOa  added)  are  only  very  slightly 
soluble  even  in  dilute  HN03.  In  this  case  the  subsequent  procedures 
(namely,  P.  106-110)  for  detecting  these  constituents  may  be  omitted. 

2.  The  color  of  the  precipitate  may  indicate  the  presence  of  certain 
constituents ;   thus  Ag2S  is  black ;   Agl,  yellow ;   AgBr,  light  yellow ; 
Ag,Fe(CN),,   orange;    AgCl,  Ag2(CN)2,  AgSCN,   and  Ag4Fe(CN)(lj 
white. 

3.  It  will  be  seen  from  the  Table  of  lonization- Values  in  the  Ap- 
pendix that  all  the  acids  of  the  chloride-group,  except  H2S,  HCN,  and 
HC1O,  are  largely  ionized.     Consequently,  their  silver  salts  would  be 
expected  to  be  only  slightly  more  soluble  in  dilute  HNOS  than  in  water ; 
and  this  is  the  case.    AgS  and  Ag2(CN)2,  however,  being  salts  of  slightly 
ionized  acids,  might  be  expected  to  dissolve  easily  in  dilute  HNO3  in 
virtue  of  the  tendency  of  its  H+  ion  to  form  unionized  H-jS  or  HCN 


P.  10S  BEHAVIOR  TOWARD  GROUP-REAGENTS  141 

with  the  S=  or  CN~  ion  of  the  salt.  That  they  do  not  so  dissolve 
arises  from  exceptional  conditions.  Ag2S  is  not  much  soluble  in 
dilute  HNOj  because  its  solubility  in  pure  water  is  so  extremely  small 
that  there  is  only  a  very  minute  concentration  of  S=  ion  in  the  saturated 
solution,  and  this  can  yield,  in  accordance  with  the  mass-action  law, 
only  a  relatively  small  concentration  of  HS~  and  unionized  HiS  with 
the  H+  ion  of  the  HNO».  Silver  cyanide  has  for  another  reason  a 
very  slight  concentration  of  its  anion  in  its  saturated  solution ;  namely 
because  this  salt  exists  mainly  as  Ag+  and  Ag(CN)j~,  and  scarcely 
at  all  as  Ag+  and  CN~  ions. 

4.  The  other  silver  salts  either  are  very  soluble  or  moderately  solu- 
ble in  water  (as  are  the  nitrate,  chlorate,  fluoride,  and  sulfate),  or  in 
neutral  solution  they  form  precipitates  which  dissolve  readily  in  dilute 
HNOj,  owing  to  displacement  by  it  of  the  less  ionized  acid  (as  do  the 
carbonate,  sulfite,  nitrite,  borate,  chromate,  oxalate,  phosphate,  arsen- 
ate,  and  arsenite). 

Procedure  103.  —  Detection  of  the  Sulfate-Group. — Dilute  2  cc. 
of  the  Na2CO3  solution  (P.  101)  with  2  cc.  of  water,  and  add 
HAc,  first  5  drops  at  a  time  till  the  mixture  reddens  litmus 
paper,  and  then  as  much  more  as  has  already  been  added. 
Filter  out  any  precipitate.  Add  i  cc.  of  BaCl2  solution  and 
3  cc.  of  (sulfate-free)  CaCl2  solution,  heat  the  mixture  nearly 
to  boiling,  and  let  it  stand  for  at  least  10  minutes.  (Precipitate, 
presence  of  SULFATE-GROUP.) 

Notes.  —  i.  In  case  no  precipitate  results,  it  shows  the  absence  of 
the  sulf ate-group ;  and  the  subsequent  Procedures  (P.  1 1  i-i  1 2)  for 
detecting  the  separate  constituents  may  be  omitted.  But  in  order 
that  this  conclusion  and  this  omission  may  be  justifiable,  it  is  necessary 
to  follow  the  directions  carefully ;  namely,  to  neutralize  fairly  exactly 
with  HAc  and  add  only  the  specified  excess,  and  to  add  the  rather  large 
quantity  of  CaCl2  solution  and  allow  the  mixture  to  stand ;  for  other- 
wise fluoride,  oxalate,  and  chromate,  when  present  in  small  quantity, 
may  fail  to  give  a  precipitate.  Moreover,  a  slight  turbidity  or  opales- 
cence  must  not  be  disregarded. 

2.  This  test  depends  on  the  following  facts  in  regard  to  solubility. 
BaSO4  is  very  slightly  soluble  in  water  and  in  dilute  solutions  of  even 
largely  ionized  acids.  BaSO3  and  BaCrO4  are  also  very  slightly  soluble 
in  water ;  but,  since  the  HSOa"  and  HCrOr  ions  are  rather  slightly 
ionized,  these  salts  are  fairly  soluble  in  solutions  of  largely  ionized  acids 
such  as  HC1  or  HN03,  but  are  not  much  dissolved  by  solutions  of  a 


142  BEHAVIOR  TOWARD  GROUP -REAGENTS          P.  105 

slightly  ionized  acid,  such  as  HAc,  in  the  presence  of  one  of  its  neutral 
salts,  such  as  NaAc.  BaF2  and  BaC204  are  considerably  soluble  in 
water,  but  CaF2  and  CaC204  are  very  slightly  soluble  in  it.  The 
solubilities  of  these  two  calcium  salts  are,  however,  increased  by  the 
presence  of  H+  ion,  though  not  very  greatly  by  the  small  concentra- 
tion of  it  existing  in  a  HAc  solution  containing  NaAc. 

3.  From  a  neutral  solution  BaCl2  would  give  precipitates  also  with 
phosphate,  arsenate,  arsenite,  borate,  and  carbonate ;  but  none  of 
these  separates  from  a  solution  containing  proper  quantities  of  HAc 
and  NaAc.  The  possibility  of  such  precipitation  fixes,  however,  a 
limit  beyond  which  the  hydrogen-ion  concentration  may  not  be  di- 
minished. 

Procedure  104.  —  Detection  of  Oxidizing  Acidic  Constituents. 

—  To  i  cc.  of  the  Na2CO3  solution  (P.  101)  add  gradually  4  cc. 
of  a  saturated  solution  of  MnCl2  in  12  n.  HC1,  and  heat  the 
mixture  nearly  to  boiling.     (Dark  brown  or  black  color,  presence 

Of     NITRATE,    NITRITE,     CHLORATE,    HYPOCHLORITE,     CHROMATE, 

PERMANGANATE,  or  FERRIC  YANIDE ;  no  brown  or  black  color, 
absence  of  all  these  constituents,  unless  in  P.  105  reducing  con- 
stituents are  found  present.) 

Notes.  —  i.  This  simple  test  depends  upon  the  fact  that  all  these 
oxidizing  acidic  constituents  convert  MnCl2  into  the  dark-colored 
MnCU. 

2.  The  test  determines  at  once  the  absence  of  all  or  the  presence  of 
one  or  more  of  the  constituents  which  can  act  as  oxidizing  agents. 
Hence,  when  it  gives  a  negative  result,  it  enables  all  the  procedures 
for  detecting  these  constituents  to  be  omitted,  unless  in  P.  105  reducing 
constituents  are  found  to  be  present.  In  that  case  a  negative  result 
is  inconclusive,  and  the  corresponding  procedures  must  not  be  omitted ; 
for  the  reducing  effect  may  counteract  the  effect  of  the  oxidizing  con- 
stituent on  the  MnCl2.  Thus,  if  nitrate  or  chlorate  were  present  in 
the  alkaline  solution  together  with  an  excess  of  sulfide  or  sulfite,  the 
latter  would  on  acidification  reduce  the  nitrate  or  chlorate  and  prevent 
it  from  oxidizing  the  MnCl2. 

'  Procedure  105.  —  Detection  of  Reducing  Acidic  Constituents. 

—  Add  i  cc.  of  the  Na2CO3  solution  (P.  101)  to  a  mixture  of 
3  cc.  of  water,  i  cc.  of  HC1,  2  drops  of  Fe(N03)3  solution,  and 
2  drops  of  K3Fe(CN)6  solution;  and  let  the  mixture  stands  or 


P.  105  BEHAVIOR  TOWARD  GROUP-REAGENTS  143 

3  minutes.     (Blue  precipitate  or  green  coloration,  presence  of 

SULFIDE,  FERROCYANIDE,  IODIDE,  SULFITE,  Or  NITRITE  ;    no  blue 

precipitate  or  green  coloration,  absence  of  all  these  constituents.) 

Notes.  —  i.  This  test  depends  on  the  facts:  (i)  that  ferricyanide 
forms  no  precipitate  with  ferric  salts ;  (2)  that  ferricyanide  is  reduced 
to  ferrocyanide  by  substances  with  even  moderate  reducing  power 
(that  is,  by  those  with  fairly  small  reduction-potentials) ;  and  (3)  that 
ferric  salts  give  a  dark-blue  precipitate  of  ferric  ferrocyanide  with 
soluble  ferrocyanides.  The  tendency  of  the  ferricyanide  to  be  re- 
duced is  greatly  increased  (since  its  reduction-potential  is  greatly 
decreased)  by  the  fact  that  in  the  presence  of  ferric  salts  the  ferro- 
cyanide-ion  is  kept  at  an  extremely  small  concentration,  owing  to  the 
very  slight  solubility  of  ferric  ferrocyanide.  In  the  case  of  sulfite 
the  ferric  salt  is  more  rapidly  reduced  than  the  ferricyanide ;  but  this 
also  results  in  the  formation  of  a  blue  precipitate,  consisting  in  this 
case  mainly  of  ferrous  ferricyanide. 

2.  Under  the  conditions  of  this  Procedure  the  test  is  delicate  enough 
to  detect  in  the  i  cc.  of  Na2CO3  solution  treated  the  presence  of  o.i  mg. 
of  any  of  the  reducing  constituents  or  about  0.1%  of  these  constituents 
in  the  substance.    Hence,  if  the  test  gives  negative  results,  all  these 
constituents  may  be  assumed  absent  and  the  subsequent  Procedures 
modified  accordingly.     Even  when  in  P.   104  oxidizing  constituents 
were  found  present,  a  negative  result  is  fairly  conclusive;    for  only 
those  oxidizing  agents,  which,  like  permanganate,  chromate,  and  hypo- 
chlorite,  are  so  powerful  as  to  be  practicably  incompatible  with  reducing 
constituents  even  in  solid  substances,  destroy  the  reducing  constituents 
rapidly  enough  to  prevent  them  from  acting  upon  the  KjFe(CN)«. 

3.  In  the  case  of  an  effect  so  slight  that  it  is  doubtful  whether  there 
is  a  green  coloration  it  is  well  to  compare  the  color  with  that  produced 
by  adding  to  i  cc.  of  pure  3  n.  Na2C03  solution  the  volumes  of  water 
and  of  reagents  named  in  the  Procedure.     If  a  red  color  results  (owing 
to  the  presence  of  thiocyanate),  the  presence  or  absence  of  a  blue  pre- 
cipitate in  the  mixture  may  be  determined  by  filtering  it. 

4.  The  K,Fe(CN)«  reagent  should  be  frequently  prepared  freshly 
from  the  crystals,  since  exposure  to  light  slowly  reduces  it  to  K«Fe(CN)6 ; 
and  the  presence  of  this  substance,  even  in  small  proportion,  must 
obviously  diminish  the  reliability  and  delicacy  of  the  test. 


144  ANALYSIS  OF  CHLORIDE-GROUP 

ANALYSIS   OF  THE   CHLORIDE-GROUP 


P.  106 


TABLE  XVI.  —  SEPARATION  OF  THE  CHLORIDE-GROUP  INTO  SUBGROUPS. 


Sodium  Carbonate  Solution  Containing  All  Acidic  Constituents. 
To  a  portion  add  Pb(N03)2  (P.  106). 


Black 
Precipitate  : 
PbS. 
(Shows 

SULFIDE.) 

Filtrate.    Add  HAc  and  Ni(N03)z  (P.  106). 

Precipitate  : 
Ni2Fe(CN)6, 
Ni3(Fe(CN)6)2, 
Ni(CN)2. 
(Shows  simple 
or  complex 

CYANIDE.) 

See  Table  XVII. 

Filtrate:    NaSCN,    Nal,    NaBr, 
NaCl,  NaClO3. 
Add  AgN03  and  HN03  (P.  107}. 

Precipitate  : 
AgSCN,  Agl, 
AgBr,  AgCl. 

(ShoWS  HAIJDE 
Or  THIOCYANATE.) 

See  Table  XVIII. 

Filtrate:  AgC103. 
Add  NaNOz 

(P.  10$). 

Precipitate  :  AgCl. 
(Shows 

CHLORATE  or 
HYPOCHLORITE.) 

Procedure  106.  —  Precipitation  of  Sulfide  and  of  the  Cyanides. 
—  In  case  in  P.  102  AgNO3  produced  a  precipitate,  treat  6  cc. 
of  the  Na2C03  solution  (P.  101)  as  follows : 

Add  5  cc.  of  water  and  i  drop  of  Pb(NO3)2  solution,  and  shake 
the  mixture.  (White  precipitate,  absence  of  SULFIDE  ;  gray  or 
black  precipitate,  presence  of  SULFIDE.)  If  a  gray  or  black  pre- 
cipitate forms,  add  Pb(N03)2  solution,  i  cc.  at  a  time,  shaking 
after  each  addition,  till  the  precipitate  begins  to  get  lighter 
colored,  not  adding  more  than  12  cc.  in  all.  Filter  out  the 
precipitate. 

To  the  nitrate  add  HAc,  10  drops  at  a  time,  till  the  mixture 
reddens  litmus  paper,  and  then  one-third  as  much  more  HAc 
as  has  already  been  added ;  and  filter  out  any  precipitate  that 
may  separate.  To  the  solution  add  3-10  cc.  of  Ni(NO3)2  solu- 
tion, and  let  the  mixture  stand  at  least  10  minutes  with  frequent 
shaking.  (Precipitate,  presence  of  CYANIDE  or  of  FERRO  or  FER- 


P.  106  ANALYSIS  OF  CHLORIDE-GROUP  145 

RICYANIDE.)  Filter  the  mixture,  preferably  with  the  aid  of  gentle 
suction  if  the  precipitate  is  large  (see  Note  6).  Wash  the 
precipitate  thoroughly.  (Filtrate,  P.  107 ;  precipitate,  P.  109.) 

Notes.  —  i.  The  addition  of  Pb(NOa)2  serves  not  only  to  detect 
sulfide,  but  to  remove  it  (as  far  as  possible),  so  that  it  may  not  pre- 
cipitate with  the  Ni(NO3)2  and  AgNO8  in  the  subsequent  operations. 
It  is  added  before  the  solution  is  acidified;  for  otherwise,  much  H2S 
would  be  lost  during  the  effervescence  caused  by  the  escape  of  the 
CO2,  and  sulfite  or  nitrite,  if  present,  would  immediately  destroy  the 
sulfide  with  liberation  of  sulfur. 

2.  The  precipitation  of  PbS  from  the  Na2COj  solution  depends  on 
the  fact  that,  though  PbCOa  is  a  very  slightly  soluble  salt,  PbS  is  very 
much  less  soluble  (see  the  Table  of  Solubilities  in  the  Appendix). 
Consequently,  conversion  of  the  former  into  the  latter  salt  takes  place, 
in  accordance  with  the  principles  presented  in  Note  7,  P.  101,  until 
the  concentration-ratio  (S=)/(CO3=)  attains  a  definite  value,  which  in 
this  case  is  of  very  small  magnitude.    A  small  quantity  of  sulfide  does, 
however,  remain  in  solution ;   and  this  commonly  gives  a  small  dark 
precipitate  when  AgNOa  is  subsequently  added  to  precipitate  the  halides. 

3.  As  most  of  the  common  sulfides  are  not  attacked  by  Na2COa 
solution,  non-formation  of  a  precipitate  does  not  show  the  absence  of 
sulfide  in  the  substance.    The  original  substance  or  the  residue  insol- 
uble in  Na2COj  solution  must  therefore  also  be  tested  for  it  by  P.  117, 
as  was  directed  in  P.  100. 

4.  Certain  substances  soluble  in  Na-iCOj  solution,  but  not  soluble 
in  the  HAc  solution,  may  precipitate  on  neutralizing  the  alkaline  solu- 
tion with  HAc ;  for  example,  antimony  or  tin  hydroxide ;  sulfur,  from 
a  persulfide  or  thiosulfate ;    H2SiO»  arising  from  silica  or  a  silicate ; 
Ni(CN)2,  Ag2(CN)2,  or  other  cyanide,  previously  held  in  solution  by 
KCN  or  NaCN. 

5.  Since  Ni(CN)2  is  slightly  soluble  in  HAc  solutions  and  since  it 
tends  to  remain  in  the  colloidal  state,  at  least  3  cc.  of  Ni(NO»)2  should 
be  added  so  as  to  diminish  its  solubility,  even  when  no  precipitate 
results  on  the  first  addition,  and  the  mixture  should  be  allowed  to 
stand  at  least  10  minutes,  and  preferably  for  a  longer  time. 

6.  The  precipitates  produced  with  cyanide  and  ferro  and  fern- 
cyanide  by  Ni(NOj)2  solution  are  slimy,  and  are  sometimes  very  diffi- 
cult to  filter.    If  this  proves  to  be  the  case,  paper-pulp,  prepared  as 
described  in  Note  4,  P.  74,  may  be  added  to  the  filtrate  and  after 
vigorous  shaking  the  mixture  again  filtered. 


146  ANALYSIS  OF  CHLORIDE-GROUP  P.  108 

Procedure  107.  —  Precipitation  of  the  Halides.  —  To  the  fil- 
trate from  the  Ni(N03)2  precipitate  (P.  106)  add  2  cc.  of  HNOs 
and  1-8  cc.  of  AgNO3  solution.  (If  there  is  a  black  precipitate, 
add  5  cc.  more  HNO3,  and  boil  the  mixture  gently  for  a  minute 
or  two.)  (White  precipitate,  presence  of  CHLORIDE  or  THIO- 
CYANATE  ;  yellow  precipitate,  presence  of  BROMIDE  or  IODIDE.) 
Filter,  and  wash  the  precipitate.  (Filtrate,  P.  108 ;  precipitate, 
P.  no.) 

Notes.  —  i.  As  to  the  solubilities  of  silver  salts  on  which  this  separa- 
tion of  the  halides  and  thiocyanate  from  other  constituents  depends, 
see  the  Notes  on  P.  102. 

2.  Since  chloride  is  a  common  impurity,  care  must  be  taken  to  use 
throughout  this  analysis  of  the  chloride-group  reagents  that  are  as 
free  as  possible  from  chloride.    And,  when  AgNOs  produces  a  small 
precipitate,  a  blank  test  should  be  made  by  mixing  with  2  cc.  of  AgNQs 
solution,  in  succession,  5-cc.  portions  of  HNOs,  of  3  n.  Na2C03  solution, 
of  HAc,  and  of  Ni(NO3)2  solution. 

3.  A  black  precipitate  may  be  produced  by  AgNOs  when  sulfide 
is  present  in  the  substance,  owing  to  the  fact  that  it  was  not  com- 
pletely precipitated  by  Pb(NO3)2  in  P.  106.     Such  a  precipitate  dis- 
solves, however,  when  more  HN03  is  added  and  the  mixture  is  heated. 

Procedure  108.  —  Detection  of  Chlorate  and  Hypochlorite.  — 
To  the  filtrate,  in  case  oxidizing  acidic  constituents  were  found 
present  in  P.  104,  add  a  few  drops  more  AgNO3  solution  and 
5  to  20  drops  of  (chloride-free)  3  n.  NaNO2  solution.  (White 
precipitate,  presence  of  CHLORATE  or  HYPOCHLORITE.) 

In  case  AgN03  produced  a  precipitate  both  before  and  after 
the  addition  of  NaNO2  solution,  treat  0.5  g.  of  the  powdered 
original  substance  with  10  cc.  of  cold  water,  filter  the  mixture, 
and  treat  the  filtrate  as  follows :  To  one-half  add  HAc,  a  few 
drops  at  a  time,  until  the  solution  is  acid ;  then  add  about  3  cc. 
of  PbAc2  solution,  heat  the  mixture  to  boiling,  and  let  it  stand 
5  minutes.  (Brown  precipitate,  presence  of  HYPOCHLORITE.)  In 
case  hypochlorite  is  found  present,  to  the  other  half  of  the 
filtrate  add  20  cc.  of  water,  5  cc.  of  HNO3,  5  cc.  of  NaAs02  solu- 
tion, and  5  cc.  of  AgN03  solution;  and  filter,  rejecting  the 


P.  108  ANALYSIS  OP  CHLORIDE-GROUP  147 

precipitate.  To  the  filtrate  add  a  few  drops  more  AgN03 
solution  and  i  cc.  of  (chloride-free)  3  n.  NaNO2  solution.  (White 
precipitate,  presence  of  CHLORATE.) 

Notes.  —  i.  The  reduction  of  chlorate  to  chloride  by  HNO»  is  so 
rapid,  even  in  the  cold,  that  0.5  mg.  of  ClOj  produces  a  precipitate  in  a 
few  seconds.  The  NaN02  may  produce  a  precipitate  of  AgNOj ;  but 
this  dissolves  on  shaking  the  mixture.  Before  the  addition  of  the 
NaNO2  a  few  drops  of  AgNO»  are  added,  to  make  sure  that  the  halides 
have  been  completely  precipitated. 

2.  The  formation  of  a  precipitate  with  NaNO2  may  arise  from  the 
presence  in  the  substance  either  of  chlorate  or  of  hypochlorite.     Since 
hypochlorite  is  changed  to  chloride  and  chlorate  by  boiling  with  Na2COj 
solution,  it  is  directed  to  test  a  fresh  sample  of  the  substance  for  hypo- 
chlorite and  chlorate  in  case  AgNOj  produced  a  precipitate  both  before 
and  after  the  addition  of  NaNO2.    This  need  be  done,  however,  only 
in  case  also  reducing  constituents  incompatible  with  hypochlorite  are 
not  present,  as  described  in  Notes  9-11  of  P.  101. 

3.  This  test  for  hypochlorite  depends  on  the  oxidation  of  the  lead 
salt  to  Pb02  by  the  unionized  HC1O,  which  is  set  free  by  the  more 
largely  ionized  HAc.    The  solution  is  acidified  with  HAc,  rather  than 
with  HNOj,  since  the  oxidation  doe*  not  take  place  in  the  presence  of 
much  hydrogen-ion.    The  test  is  not  made  in  the  unneutralized  solu- 
tion, even  though  it  would  then  be  somewhat  more  delicate,  because 
in  the  presence  of  hydroxide-ion  peroxide  and  ferricyanide  also  oxidize 
lead  salts  to  Pb02. 

4.  In  the  chlorate  test  the  NaAs02  added  reduces  the  hypochlorite 
immediately  to  chloride,  but  does  not  affect  the  chlorate.    After  re- 
moving with  AgNO3  the  chloride  so  produced,  the  chlorate  is  reduced 
to  chloride  by  NaNO2. 


148  ANALYSIS  OF  CHLORIDE-GROUP  P.  109 

TABLE  XVII.  —  DETECTION  OF  THE  SEPARATE  CYANIDES. 

Nickel  Precipitate:  Ni2Fe(CN)6,  Ni3(Fe(CN)6)2,  Ni(CN)2. 
Add  NHtOH  (P.  109). 

Solution:   (NH3)4Ni(OH)2,  (NH4)4Fe(CN)6,  (NH4)3Fe(CN)6j  NH4CN. 
Add  AgN03  and  Na*S03. 


Precipitate:  Ag4Fe(CN)6. 
Add  HCl  and  Fe(NOJ3. 


Filtrate:     NH4Ag(CN)2,    Ni(NO3)2)   AgNO3, 
and  NH4N03. 

Add  HN03. 


Blue  residue  : 
Fe4(Fe(CN)6)3 
and  AgCl. 
(Shows  FERRO  or 

FERRICYANIDE.) 

Precipitate:  Ag2(CN)2. 
Add  (NHJtS. 

Filtrate  : 
Ni,  Ag,  and 
NH4  nitrates. 
Reject. 

Residue:  A&S. 
Reject. 

Solution  : 
NH4CNS. 
Add  Fe(N03)3. 

Red  color  : 
Fe(CNS)3. 

(Shows  CYANIDE.) 

Procedure  109.  —  Detection  of  the  Di/erent  Cyanides.  —  Pour 
two  or  three  times  through  the  filter  containing  the  Ni(NOs)2 
precipitate  (P.  106)  a  lo-cc.  portion  of  3  n.  NH4OH,  and  add 
to  the  mixture  2-5  cc.  of  AgN03  solution,  then  Na2S03  solution, 
a  few  drops  at  a  time  till  any  brown  color  disappears,  shaking 
after  each  addition.  (White  precipitate,  presence  of  FERRO  or 
FERRICYANIDE.)  Filter  the  mixture. 

Treat  the  precipitate  on  the  filter  with  a  mixture  of  i  cc.  of 
HCl  and  i  cc.  of  Fe(N03)3  solution.  (Blue  residue,  presence  of 

FERRO  or  FERRICYANIDE.) 

To  the  filtrate  from  the  AgN03  precipitate  add  HN03  grad- 
ually till  the  odor  of  NHa  disappears,  then  5  cc.  more.  (White 
precipitate,  presence  of  CYANIDE.)  Filter,  rejecting  the  filtrate. 
Pour  repeatedly  through  the  filter  a  5-cc.  portion  of  (NH4)2S 
reagent,  evaporate  the  solution  just  to  dryness,  and  add  to  the 
residue  2  cc.  of  HCl  and  2  cc.  of  Fe(N03)3  solution.  (Red 
color,  presence  of  CYANIDE.) 


P.  109  ANALYSIS  OF  CHLORIDE-GROUP  149 

In  case  it  is  found  that  either  ferro  or  ferricyanide  is  present, 
to  i  cc.  of  the  Na2COs  solution  (P.  101)  add  5  cc.  of  water, 
i  cc.  of  HNO3,  and  i  cc.  of  Fe(NO3)3  solution.  (Dark  blue 
precipitate,  presence  of  FERROCYANIDE.)  Filter  the  mixture,  re- 
peatedly if  necessary;  and  add  to  the  nitrate  i  cc.  of  FeCl2 
solution.  (Dark  blue  precipitate,  presence  of  FERRICYANIDE.) 

Notes.  —  i.  This  separation  of  ferro  and  ferricyanide  from  cyanide 
depends  upon  the  fact  that  the  cyanide  of  silver  is  moderately  soluble 
in  NH4OH,  while  the  ferrocyanide  is  only  very  slightly  soluble  in  it. 
Silver  ferricyanide  is  also  moderately  soluble  in  NH4OH,  but  it  is 
reduced  to  the  ferrocyanide  by  the  Na2SO8  added. 

2.  It  is  not  practicable  to  separate  ferro  and  ferricyanide  by  filter- 
ing out  the  silver  ferrocyanide  before  adding  Na2SOs  and  then  adding 
this  reagent  to  the  filtrate,  for  the  reason  that  the  ferricyanide  is  re- 
duced, at  least  partially,  by  the  ammoniacal  nickel  solution  alone.    It 
is  desirable  to  confirm  the  presence  of  ferro  or  ferricyanide  by  treating 
the  AgNOs  precipitate  with  Fe(NOa)3,  since  that  precipitate  may  con- 
sist of  Ag2(CN)2,  which  is  only  moderately  soluble  in  dilute  NH4OH. 

3.  Owing  to  the  possible  presence  of  a  little  chloride  arising  from 
contamination  or  from  incomplete  washing  of  the  Ni(NO»)2  precipitate, 
it  is  essential  to  confirm  the  presence  of  cyanide  in  any  precipitate  pro- 
duced by  the  HNOj.    The  confirmatory  test   depends  upon  the  fact 
that  Ag2(CN)2  is  metathesized  by  (NH4)2S,  yielding  Ag2S  as  a  residue 
and  NH4CN  in  solution,  and  that  the  latter  is  converted  during  the 
evaporation  by  the  liberated  sulfur  into  NH4CNS,  which  then  gives 
a  red  color  with  the  ferric  salt. 

4.  The  process  for  distinguishing  ferro  and  ferricyanide  described 
in  the  last  paragraph  of  the  Procedure  is  based  on  the  following  facts. 
With  ferrocyanide  ferric  salts  give  a  dark  blue  precipitate  of  ferric 
ferrocyanide  (Prussian  blue),  while  with  ferricyanide  they  give  no 
precipitate.    With  ferrocyanide  ferrous  salts  give  a  precipitate  (of 
ferrous  ferrocyanide),  which  is  white  if  no  ferric  salt  is  present,  but 
which,  owing  to  slight  oxidation,  is  usually  light-blue ;  with  ferricyanide 
ferrous  salts  give  a  dark-blue  precipitate,  consisting  mainly  of  ferrous 
ferricyanide. 


ANALYSIS  OF  CHLORIDE-GROUP 


P.  110 


TABLE  XVIII.  —  DETECTION  OF  THIOCYANATE,  IODIDE,  BROMIDE,  AND 
CHLORIDE. 


Silver  Precipitate :  AgSCN,  Agl,  AgBr,  AgCl. 

Treat  with  NH^OH  and  (NH<}£  (P.  no). 


Residue : 


Solution:  NH4SCN,  NHJ,  NH4Br,  NH4C1. 
Add  HN03,  Fe(N03)3,  and  CC14. 


CC14 layer:  I2. 
(Purple  color 
shows  IODIDE.) 


Water  layer:  I2,  HBr,  HC1,  Fe(SCN)3. 
(Red  color  shows  THIOCYANATE.) 
Boil;  then  cool  and  add  KMnO*  and  CCh. 


Vapor: 
I*. 


CC14  layer : 

Br2. 
(Orange  color 

Shows  BROMIDE.) 


Water  layer : 
Br2,  HC1,  H2S04. 
Boil;  then  add  AgNOz. 


Vapor: 
Br2. 


Precipitate : 

AgCl. 
(Shows 

CHLORIDE.) 


Procedure  no.  —  Detection  of  Thiocyanate  and  the  Separate 
Halides.  —  Transfer  the  AgNO3  precipitate  (P.  107)  to  a  small 
casserole  (see  Note  i,  P.  22) ;  and  treat  it  with  5  cc.  of  15  n. 
NH4OH.  Add  (NH4)2S  reagent,  10  drops  at  a  time,  till,  after 
heating  the  mixture  nearly  to  boiling  and  letting  the  precipitate 
settle,  the  reagent  produces  no  further  precipitate.  Filter  out 
and  reject  the  precipitate. 

Evaporate  the  nitrate  till  it  no  longer  smells  of  ammonia, 
add  5  cc.  of  water,  and  filter  out  any  precipitate.  Pour  the 
solution  into  a  small  separating  funnel,  add  i  cc.  of  HN03, 
3-8  cc.  of  Fe(NO3)3  solution,  and  i  cc.  of  CCU  (carbon  tetra- 
chloride),  and  shake  the  mixture  for  a  minute  or  two.  (Purple 
color  of  the  carbon-tetrachloride  layer,  presence  of  IODIDE; 
red  color  of  the  aqueous  layer,  presence  of  THIOCYANATE  or  of 
much  IODIDE.) 


P.  110  ANALYSIS  OF  CHLORIDE-GROUP  151 

In  case  iodide  is  absent,  proceed  as  described  in  the  next  to 
last  paragraph  of  this  Procedure. 

In  case  iodide  is  present,  draw  off  the  CCU,  add  3  cc.  of  fresh 
CCU,  and  shake  the  mixture ;  repeating  these  operations  till 
the  CCLt  layer  no  longer  has  a  dark  purple  color.  (Red  color 
in  the  aqueous  layer,  presence  of  THIOCYANATE.)  Transfer  the 
aqueous  layer  to  a  casserole,  boil  it  for  one  minute,  cool  the 
mixture,  pour  it  into  a  separating  funnel,  add  i  cc.  of  CCU, 
shake  the  mixture,  and  treat  it  as  follows. 

Add  to  the  mixture  in  the  separating  funnel  2  cc.  of  HNOa, 
and  then  0.2  n.  KMnC>4  solution,  2  drops  at  a  time,  till  the 
aqueous  layer  becomes  purple.  (Yellow  or  orange  color  in  the 
carbon-tetrachloride  layer,  presence  of  BROMIDE.) 

Transfer  the  aqueous  layer  to  a  flask,  dilute  it  to  about  40  cc. 
with  water;  and,  in  case  bromide  or  thiocyanate  is  present, 
boil  the  mixture  for  5  minutes,  adding  more  0.2  n.  KMnCX  if 
the  mixture  loses  its  purple  color  (which  should  be  pronounced 
enough  to  be  noticeable  even  though  a  brown  precipitate  has 
separated).  Add  to  the  mixture  3  n.  (chloride-free)  NaNOz 
solution,  2-3  drops  at  a  time,  till  the  mixture  is  decolorized,  and 
any  precipitate  has  dissolved.  Then  add  1-5  cc.  of  AgNOs 
solution.  (White  precipitate,  presence  of  CHLORIDE.) 

Notes.  —  i.  The  AgNOj  precipitate  is  treated  first  with  NH4OH, 
and  not  directly  with  (NH4)2S  reagent,  because  the  NH4OH  dissolves 
the  silver  precipitate  wholly  or  in  part,  and  thus,  by  diminishing  the 
extent  to  which  the  particles  become  coated  with  Ag2S,  makes  the 
metathesizing  action  of  the  (NH4)2S  reagent  more  rapid.  The  use  of 
NH4OH  also  facilitates  the  manipulation,  making  it  easier  to  remove 
the  paper  from  the  mixture. 

2.  The  5  cc.  of  15  n.  NH4OH  dissolves  the  maximum  quantity  of 
AgCl  that  may  be  present,  but  only  a  small  quantity  of  AgSCN  or 
AgBr,  and  scarcely  any  Agl.    The  behavior  towards  NH4OH  may 
therefore  indicate  the  character  of  the  halide  present. 

3.  The  greatly  increased  solubility  of  these  silver  salts  in  NH4OH 
is  due  to  the  formation  of  complex  silver-ammonia  cations,  of  which 
Ag(NH5)j+  is  the  one  that  is  mainly  produced,  so  long  as  the  NI^OH 
is  only  moderately  concentrated;   and  under  these  conditions  it  can 
readily  be  shown,  by  combining  the  mass-action  expression  for  the 


152  ANALYSIS  OF  CHLORIDE-GROUP  P.  110 

dissociation  of  this  complex  ion  with  the  solubility-product  expression 
for  the  silver  salt,  that  the  solubility  of  a  silver  salt  in  NH4OH  solution 
is  proportional  to  its  solubility  in  water  (provided  this  is  small)  and 
to  the  concentration  of  the  NH4OH. 

4.  That  even  so  slightly  soluble  a  substance  as  Agl  is  almost  com- 
pletely metathesized  by  (NH4)2S  is  due  to  the  extraordinarily  small 
solubility  of  Ag2S. 

5.  It  is  important  to  use  nearly  colorless  (NH^S  reagent  of  stand- 
ard concentration,  which  has  not  been  decomposed  by  standing;   for 
otherwise  the  large  quantity  of  sulfur  that  separates  during  the  evapo- 
ration and  the  reducing  action  of  non-volatile  sulfur  acids  present  as 
impurities  will  interfere  with  the  detection  of  the  halides. 

6.  The  fact  that  iodide  is  the  only  one  of  the  three  halides  that 
reduces  ferric  salts  (with  liberation  of  the  halogen)  is  due  to  the  values 
of  the  specific  reduction-potentials  of  the  three  halide  ions  in  relation 
to  that  of  ferrous  ion.    Thus  by  reference  to  the  Table  in  the  Appendix 
it  is  seen  that  Fe++,  Fe+++  has  a  smaller  specific  reduction-potential 
than  I~,  I2,  but  a  much  larger  one  than  Br~,  Br2,  or  Cl~,  C12. 

7.  The  addition  of  CCLi  to  the  mixture  serves  the  double  purpose 
of  making  the  test  for  iodide  more  delicate  and  characteristic,  and  of 
removing  most  of  the  free  I2  from  the  aqueous  solution  and  thus  enabling 
thiocyanate  to  be  detected  by  the  red  color  which  it  produces  with 
ferric  salts. 

8.  The  quantity  of  I2  or  Br2  that  passes  from  the  water  layer  into 
the  CCU  layer  is  determined  by  the  so-called  distribution-law.    Ac- 
cording to  this  law,  at  any  given  temperature,  after  equilibrium  is 
reached,  a  substance  distributes  itself  between  two  non-miscible  sol- 
vents in  such  proportions  that  the  concentration  of  the  substance  in 
one  solvent  bears  a  definite  ratio,  called  the  distribution-ratio,  to  its 
concentration  in  the  other  solvent,  whatever  may  be  the  relative 
quantities  of  the  two  solvents  taken  or  the  quantity  of  the  substance 
originally  present  in  either  of  them.    The  value  of  this  ratio  varies 
with  the  nature  of  the  substance,  the  nature  of  the  two  solvents,  and 
the  temperature.     At  25°  its  value  for  I2  between  carbon  tetrachloride 
and  water  is  85 ;   and  its  value  for  Br2  between  the  same  solvents  is 
23.    This  signifies  that  I2  will  pass  into  the  CCU  layer  from  the  water 
layer  until  its  concentration  (that  is,  the  quantity  of  it  per  unit-volume) 
is  85  tunes  as  great  in  the  CCU  layer  as  it  is  in  the  water  layer ;  and 
that  Br2  will  so  pass  till  its  concentration  in  the  CCL,  layer  is  23  tunes 
as  great  as  in  the  water  layer. 

9.  The  red  color  produced  by  ferric  salts  with  thiocyanate  is  due 
to  the  formation  of  Fe(SCN)*,  which  is  less  ionized  than  most  salts  of 


P.  110  ANALYSIS  OF  CHLORIDE-GROUP  153 

the  same  valence  type.  The  color  is  pronounced  even  with  0.1-0.2  mg. 
of  SCN ;  and  the  much  less  intense  color  produced  by  free  I2  will  not 
be  mistaken  for  it,  provided  this  has  been  so  far  removed  from  the 
aqueous  solution  that  the  CCU  layer  that  has  been  shaken  with  it  is 
pink,  not  a  dark  purple. 

10.  After  the  presence  or  absence  of  thiocyanate  is  determined,  the 
remaining  I2  is  expelled  by  boiling,  since  this  removes  it  much  more 
rapidly  than  continued  extraction  with  CCU.    The  mixture  must  not 
be  boiled,  however,  before  the  presence  or  absence  of  thiocyanate  has 
been  determined,  since  Fe(SCN)3  is  decomposed  by  heating  with  I2. 
The  iodine  must  be  completely  removed,  since  even  a  small  quantity 
would  obscure  the  test  for  bromide.     The  solution  is  shaken  with  a 
fresh  portion  of  CCU  before  the  Br2  is  liberated  by  KMnO4,  to  make 
sure  that  the  solution  is  absolutely  free  from  I2. 

11.  In  a  cold  moderately  acid  solution  Br2,  but  not  C12,  is  liberated 
by  KMnO4  from  the  corresponding  halicle.     This  difference  is  not  due, 
as  is  the  different  behavior  of  iodide  and  bromide  toward  ferric  salts, 
to  an  intermediate  value  of  the  reduction-potential  of  the  manganese 
compounds ;    for  the  value  of  this  potential,  though  not  well  known, 
is  in  acid  solution  undoubtedly  much  smaller  even  than  that  of  Cl~, 
C12.    The  difference  in  this  case  arises  primarily,  not  from  difference 
in  the  equilibrium  conditions,  but  from  the  fact  that  the  rate  of  the 
reaction  between  KMnO4  and  bromide  is  very  much  greater  than  the 
rate  of  the  reaction  between  KMnO4  and  chloride,  at  the  same  tem- 
perature and  with  the  same  concentrations  of  the  reacting  substances. 
Thus  under  the  conditions  prevailing  when  the  KMnO4  is  first  added, 
namely,  at  room  temperature  in  a  solution  i  n.  to  1.5  n.  in  HNO3,  the 
rate  of  the  bromide  reaction  is  so  large  that  most  of  the  Br2  is  liberated 
almost  immediately,  while  the  rate  of  the  chloride  reaction  is  negligible. 

12.  The  rate  of  both  these  reactions  is  very  greatly  increased  by 
increase  of  temperature  and  by  increase  in  the  hydrogen-ion  concen- 
tration.   Thus  at  the  boiling  temperature  the  rate  of  the  chloride 
reaction  would  be  fairly  large  in  a  solution  i  n.  to  1.5  n.  in  HNOj- 
Therefore,  before  the  solution  is  boiled  to  expel  the  free  Br2,  it  is  diluted 
to  about  40  cc.  with  water,  so  as  to  reduce  the  hydrogen-ion  concen- 
tration, and  thus  compensate  the  effect  of  the  higher  temperature. 
In  the  Procedure  the  HN03  concentration  is  so  adjusted  that  even 
a  small  quantity  of  bromide  is  decomposed  quickly  in  the  cold,  and 
only  a  small  quantity  of  any  chloride  present  is  acted  upon  in  the 
boiling  solution. 

13.  The  foregoing  facts  illustrate  three  general  principles  in  regard 
to  the  rate  of  chemical  reactions :  first,  that  the  rate,  under  given  con- 


154  ANALYSIS  OF  CHLORIDE-GROUP  P.  110 

ditions  of  temperature  and  concentration,  varies  within  the  widest 
possible  limits  with  the  nature  of  the  chemical  substances  involved; 
second,  that  the  rate  with  the  same  chemical  substances  at  given 
concentrations  increases  very  rapidly  with  rising  temperature,  1000 
fold  as  much  time  being  often  required  to  produce  a  given  amount  of 
change  at  20°  as  at  100° ;  and  third,  that  the  rate  with  the  same  chemical 
substances  at  a  given  temperature  is  increased  by  increasing  the  con- 
centration of  any  of  the  reacting  substances,  and  in  a  higher  degree  for 
any  substance  of  which  a  relatively  large  number  of  molecules  are  in- 
volved in  the  reaction.  By  writing  the  equation  for  the  ionic  reaction 
between  bromide,  permanganate,  and  any  acid,  8  H+  will  be  seen  to 
react  with  i  Mn04~,  thus  explaining  the  great  effect  of  the  concen- 
tration of  the  hydrogen-ion  on  the  rate  of  the  reaction. 

14.  Even  I  mg.  of  bromide  yields  enough  Br2  to  impart  a  noticeable 
yellow  tinge  to  the  i  cc.  of  CCU.     A  small  quantity  of  this  solvent  is 
used  so  as  to  increase  the  Br2  concentration  hi  it. 

15.  On  boiling  the  mixture  containing  the  KMnO<  a  brown  precipi- 
tate of  hydrated  MnO»  results  when  much  bromide  is  present;   for 
when,  as  in  this  case,  a  solution  has  only  a  moderate  hydrogen-ion 
concentration,  HMnO«  may  not  be  wholly  converted  by  reducing  sub- 
stances to  manganous  salt,  but  may  be  partially  reduced  to  the  inter- 
mediate stage  represented  by  the  brown  precipitate.     On  the  subse- 
quent addition  of  HNOj  this  substance,  as  well  as  any  excess  of  KMnO<, 
is  instantaneously  reduced  and  a  colorless  mixture  results. 

16.  The  presence  of  thiocyanate  does  not  interfere  with  the  test 
for  chloride;  for  it  is  instantly  destroyed  (converted  into  sulfate)  by 
the  KMn04,  before  the  AgNO3  is  added. 

17.  A  small  precipitate  of  AgCl  obtained  at  the  end  of  the  Procedure 
does  not  necessarily  show  the  presence  of  chloride  in  the  substance, 
unless  the  reagents  have  been  proved  to  be  entirely  free  from  chloride. 
A  blank  test  with  the  reagents  should  therefore  be  made  in  any  doubt- 
ful case;    and  a  turbidity  should  be  compared  with  the  precipitate 
produced  by  £  mg.  of  chloride,  to  determine  whether  it  is  really  sig- 
nificant. 


p.  Ill 


ANALYSIS  OF  SULFATE-GROUP 
ANALYSIS   OF   THE   SULFATE-GROUP. 


155 


TABLE  XIX.  —  DETECTION  OF  SULFATE,  SULFITE,  CHROMATE, 
FLUORIDE,  AND  OXALATE. 

Sodium  Carbonate  Solution  Containing  All  Acidic  Constituents. 
Acidify  with  HCl,  and  add  BaCl2  (P.  in). 


Precipitate  : 
BaSO4. 
(Shows 

SULFATE.) 

Filtrate:  Na2SO3,  Na2Cr2O7,  NaF,  Na2C2O4,  BaCl2.   Add  Brt. 

Precipitate  : 
BaS04. 
(Shows 

SULFITE.) 

Filtrate:  Na2Cr2O7,  NaF,  Na2C2O4,  BaCl2. 
Add  NaAc  and  CaCl* 

Yellow  precipitate  :    BaCrO4 
White  precipitate  :   CaF2,  CaC&*. 
Treat  portions  as  follows  : 

Heat  with  SiO2 
and  HiSOt  (P.  112). 

Dissolve  in  HN03, 
add  KMn04,  distil. 

Gas:  SiF4. 
Test  with  water. 

Vapors:   C02. 
Collect  in  Ba(OE\. 

Turbidity:  H2Si03. 

(Shows  FLUORIDE.) 

Precipitate:   BaCO3. 

(Shows  OXALATE.) 

Procedure  HI.  —  Detection  of  Sulfate,  Sulfite,  Chromate,  Fluor- 
ide, and  Oxalate.  —  In  case  in  P.  103  BaCl2  and  CaCl2  produced 
a  precipitate,  treat  6  cc.  of  the  Na^COs  solution  (P.  101)  as 
follows  (first  adding  2-10  cc.  of  AgN03  solution,  shaking  the 
mixture,  and  filtering  out  the  precipitate,  in  case  in  P.  106 
sulfide,  or  in  P.  no  thiocyanate,  was  found  present). 

Slightly  acidify  the  solution  with  HCl,  adding  it  10  drops  at 
a  time  till  the  solution  reddens  litmus  paper.  Filter  out  and 
reject  any  precipitate.  To  the  filtrate  add  just  i  cc.  of  HCl 
and  5  cc.  of  BaCl2  solution,  and  let  the  mixture  stand  hi  the 
cold  2  or  3  minutes.  (White  precipitate,  presence  of  SULFATE.) 
Filter  out  and  reject  the  precipitate. 

To  the  nitrate  add  at  once  saturated  Br2  solution,  i  cc.  at  a 


156  ANALYSIS  OF  SULFATE-GROUP  P.  Ill 

time,  till  the  liquid  after  shaking  smells  of  it,  and  heat  the  mix- 
ture nearly  to  boiling.  (White  precipitate,  presence  of  SULFITE.) 
Filter  out  and  reject  the  precipitate. 

To  the  filtrate  add  10  cc.  of  3  n.  NaAc  solution  and  10  cc. 
of  CaCl2  solution,  and  let  the  mixture  stand  at  least  15  minutes. 
(Yellow  precipitate,  presence  of  CHROMATE;  white  precipitates 
presence  of  FLUORIDE  or  OXALATE.)  Shake  the  mixture  vigorously 
so  as  to  suspend  the  precipitate,  and  pour  one-half  of  it  through 
each  of  two  niters.  Reject  the  filtrates.  Wash  the  precipitates 
thoroughly. 

Treat  one  portion  of  the  precipitate  by  P.  112,  to  determine 
the  presence  of  fluoride. 

Treat  the  other  portion  of  the  precipitate,  to  determine  the 
presence  of  oxalate,  as  follows.  Pour  repeatedly  through  the 
filter  containing  it  a  5  cc.  portion  of  hot  HNO3.  Arrange  a  dis- 
tilling apparatus  as  described  in  the  first  paragraph  of  P.  117. 
Pour  into  the  distilling  flask  through  the  safety- tube  the  HNO3 
solution,  and  also  a  5  cc.  portion  of  0.2  n.  KMnC>4  solution  which 
has  been  previously  acidified  with  HNOs  and  heated  to  boiling. 
Boil  the  contents  of  the  flask  for  2  or  3  minutes.  (White  pre- 
cipitate in  the  Ba(OH)2  solution,  presence  of  OXALATE.) 

Notes.  —  i.   As  to  the  solubilities  of  barium  and  calcium  salts  on 
which  this  method  of  analysis  depends,  see  the  Notes  on  P.  103. 

2.  AgNOs  is  added  in  case  sulfide  or  thiocyanate  is  present  in  order 
to  remove  these  constituents,  which  otherwise  would  be  oxidized  by 
the  Br2  with  the  formation  of  precipitates,  namely,  of  S  in  the  case  of 
sulfide  and  of  BaSO4  in  that  of  thiocyanate. 

3.  In  the  presence  of  the  quantities  of  HAc  and  NaAc  prescribed  in 
the  Procedure,  BaCl2  alone  would  yield  a  precipitate  with  fluoride  only 
when  more  than  10  mg.  of  F  is  present,  and  with  oxalate  only  when 
more  than  5  mg.  of  C2O4  is  present ;  but  CaCl2  produces  a  cloudiness 
with  5  mg.  of  either  of  these  constituents  within  15  minutes. 

4.  The  NaAc  and  CaCl2  solutions  used  as  reagents  must  be  free 
from  sulfate,  since  otherwise  a  precipitate  of  BaSO4  will  be  obtained  in 
the  fluoride-oxalate  test.    These   reagents  should  be   tested  in  ad- 
vance with  BaCl2  for  this  impurity;   and,  if  found  present,  it  should 
be  removed  by  adding  a  little  BaCl2  solution  to  the  reagent,  heating  to 
boiling,  and  filtering. 


P.  112  ANALYSIS  OF  SULFATE-GROUP  157 

5.  Thiosulfate  (S2OJ=)  is  a  somewhat  rare  constituent  of  industrial 
products  which,  when  treated  by  this  Procedure,  would,  like  sulfite, 
yield  a  precipitate  of  BaSO4  on  addition  of  Br2  solution.  When  present 
in  considerable  amount,  it  shows  itself  by  producing  a  precipitate 
when  the  Na2CO3  solution  is  acidified  with  HC1,  since  it  rapidly  de- 
composes into  sulfur  and  sulfite  under  the  catalytic  influence  of  a  con- 
siderable concentration  of  hydrogen-ion. 

Procedure  112.  —  Confirmatory  Test  for  Fluoride.  —  Roll  up  the 
filter  containing  the  CaCl2  precipitate  (P.  in),  wind  a  platinum 
wire  around  it,  and  heat  it  till  it  is  completely  incinerated,  allow- 
ing the  ash  to  fall  on  to  a  watch-glass.  Mix  intimately  with  the 
ash,  or  with  a  portion  of  it  if  it  is  large,  2  or  3  times  its  volume 
of  finely  powdered  quartz  (not  artificially  prepared  silica) ;  and 
transfer  it  with  the  aid  of  a  piece  of  smooth  paper  to  a  dry  test- 
tube,  about  100  mm.  in  length  and  12  mm.  in  bore.  Add  from  a 
dropper  enough  95%  H2SC>4  to  make  a  thin  paste,  taking  care 
not  to  wet  the  sides  of  the  tube.  Insert  in  the  tube  a  somewhat 
narrower  glass  tube,  wet  on  the  inside  but  dry  on  the  outside, 
so  that  it  extends  to  within  3  cm.  of  the  bottom,  supporting  it 
at  the  proper  height  by  a  rubber  band  or  stopper.  Heat  the 
mixture  carefully  over  a  small  flame  (not  enough  to  vaporize 
the  HaSCU)  for  a  minute  or  two.  (White  precipitate,  in  the 
wet  part  of  the  inner  tube,  presence  of  FLUORIDE.) 

Notes.  —  i.  This  confirmatory  test  depends  on  the  fact  that  the 
HF  liberated  from  the  CaF2  by  the  H2SO4  reacts  with  the  SiO2  with  the 
formation  of  gaseous  SiF4,  and  on  the  fact  that  this  gas  on  coming  into 
contact  with  water  reacts  with  it,  precipitating  H2SiO3  and  leaving 
H2SiF»  (fluosilicic  acid)  in  solution. 

2.  Great  care  must  be  used  to  have  the  test-tube  and  the  materials 
perfectly  dry,  and  concentrated  (95%)  H2SO4  must  be  employed ;  for 
otherwise  the  SiF4  will  be  decomposed  before  it  comes  into  contact 
with  the  wet  walls  of  the  inner  tube. 

3.  The  SiO2  used  should  be  in  the  form  of  powdered  quartz,  not  of 
precipitated  and  ignited  silicic  acid;   for  the  test  is  far  less  delicate 
with  the  latter  material,  since  it  retains  much  of  the  fluorine,  apparently 
in  the  form  of  SiOF,. 


DETECTION  OF  NITRATE  AND  NITRITE         P.  IIS 


DETECTION   OF   OTHER   CONSTITUENTS   IN   THE   SODIUM 
CARBONATE   SOLUTION 


TABLE  XX. —  DETECTION  OF  NITRATE,  NITRITE,  BORATE,  ARSENATE, 
AND  ARSENITE. 


Sodium  Carbonate  Solution  Containing  All  the  Acidic  Constituents. 
Treat  portions  as  follows : 


Boil  with 
NaOH  and  Al 
(P.  uj). 

Add  HAc  and 
CSNiH* 
(P.  H4). 

Add  HCl, 
CiHtOH, 

and  turmeric 
(P.  US)- 

Add  HCl,  NH4OH,  and 
Mg(N03)2  (P.  116). 

Precipitate  : 
MgNH4As04. 
Treat  with 
AgNOz. 

Filtrate  : 
NH4As02. 
Pass  in 
H2S. 

Vapor:  NH3. 
Test  with 
KJIgli. 

Gas: 

N2. 

Solution  : 
NH4SCN. 
Add  FeCh. 

Orange  color. 
(Shows 

BORATE.) 

Red 
precipitate  : 
HgO'HgNHjI. 
(Shows 

NITRATE  01 
NITRITE.) 

Red  color  : 
Fe(SCN)3. 

Red 

residue  : 
Ag3As04. 
(Shows 

ARSENATE.) 

Yellow 
precipitate, 
As2S3. 
(Shows 

ARSENITE.) 

(Show  NITRITE.) 

Procedure  113.  —  Detection  of  Nitrate  or  Nitrite.  —  In  case 
in  P.  104  oxidizing  acidic  constituents  have  been  found  present, 
place  2  cc.  of  the  Na^COs  solution  (P.  101),  10  cc.  of  water,  and 
3  cc.  of  NaOH  solution  in  a  50-0:.  round-bottom  flask.  (See 
Note  4.)  (In  case  in  P.  91  ammonium  was  found  present,  boil 
the  mixture  till  one-third  of  it  has  distilled  off,  and  cool  it.) 
Add  to  the  mixture  i  cc.  of  aluminum  turnings.  Hold  in  the 
vapors  a  glass  rod  wet  with  K2HgI4  reagent,  and  heat  the  mix- 
ture gently  so  as  to  keep  up  a  brisk  evolution  of  hydrogen. 
(Orange  or  red  precipitate  on  the  rod,  presence  of  NITRATE  or 
NITRITE.)  If  a  reddish  precipitate  forms,  in  order  to  estimate 
the  quantity  of  nitrate  or  nitrite  present,  insert  at  once  in  the 
neck  of  the  flask  a  rubber  stopper  fitted  with  a  delivery-tube 
leading  to  the  bottom  of  a  test-tube  containing  5  cc.  of  water 
placed  in  a  beaker  of  cold  water,  and  distil  slowly  till  about  one- 


P.  114         DETECTION  OF  NITRATE  AND  NITRITE  159 

third  of  the  liquid  has  passed  over.  To  the  distillate  add  K2HgI4 
reagent,  a  few  drops  at  a  time,  so  long  as  the  precipitate  increases. 
(Orange  or  red  precipitate,  presence  of  NITRATE  or  NITRITE.) 

Notes.  —  i.  Both  nitrate  and  nitrite  are  reduced  to  NH3  in  alkaline 
solution  by  metals  which  evolve  hydrogen.  The  NH3  produced  is 
driven  out  of  the  solution  by  boiling,  and  is  tested  for  as  in  the  Procedure 
for  the  detection  of  ammonium  (P.  91),  with  which  this  nitrate-nitrite 
test  may  be  combined  if  desired. 

2.  As  to  the  composition  of  the  K2HgI4  reagent  and  the  precipitate 
produced  by  it  with  NH8,  see  Notes  2  and  3,  P.  91. 

3.  To  estimate  the  quantity  of  nitrate  or  nitrite  present,  the  pre- 
cipitate produced  by  the  K2HgI4  reagent  may  be  compared  with  that 
produced  by  adding  the  reagent  directly  to  known  solutions  of  NH4C1, 
taking  into  account  the  fact  that  i  mg.  of  NH4  corresponds  to  about 
3  mg.  of  NO2  or  N03. 

4.  Cyanide,  ferro  and  ferricyanide,  and  thiocyanate  also  yield  NHi 
when  treated  by  this  Procedure.    In  case  in  P.  106-110  any  of  these 
constituents  was  found  present,  the  diluted  Na2C03  solution  should 
be  shaken  with  about  0.5  cc.  of  solid  Ag2CO3  and  the  precipitate  filtered 
out,  before  adding  the  NaOH  solution. 

Procedure  114.  —  Detection  of  Nitrite.  —  In  case  in  P.  113 
nitrate  or  nitrite  is  found  present,  pour  i  cc.  of  the  Na2CO3 
solution  (P.  101)  into  a  test-tube,  and  add  gradually  i  cc.  of 
HAc.  (See  Note  2.}  Then  add  i  cc.  of  a  10%  solution  of 
thiourea  (CSN2H4),  and  let  the  mixture  stand  five  minutes. 
(Formation  of  gas  bubbles,  indication  of  NITRITE.)  Add  i  cc. 
of  HC1  and  i  cc.  of  Fe(NO3)3  solution.  (Red  color,  presence 
of  NITRITE  ;  no  red  color,  presence  of  NITRATE.) 

Notes.  —  i.  The  nitrite  test  is  based  on  the  following  reaction  which 
takes  place  in  solutions  with  small  hydrogen-ion  concentrations : 

CS(NH2)2  +  HONO  =  N,  +  HSCN  +  2  H20. 

2.  No  other  constituent  gives  rise  to  HSCN ;  but,  in  case  thiocyanate 
or  iodide  is  present  in  the  substance,  as  shown  in  P.  no,  it  would  pro- 
duce a  red  color  with  Fe(N03)s.     In  that  case  it  must  be  removed  by 
shaking  the  Na2C03  solution  with  about  0.5  cc.  of  solid  Ag2CO3  and 
filtering  out  the  residue,  before  adding  the  HAc  and  thiourea. 

3.  No  provision  is  made  for  the  detection  of  nitrate  in  the  presence 
of  nitrite,  since  no  satisfactory  qualitative  method  is  known. 


160  DETECTION  OF  BORATE  P.  lie 

Procedure  115.  —  Detection  of  Borate.  —  Add  to  just  3  cc. 
of  the  Na2CO3  solution  (P.  101)  just  8  cc.  of  12  n.  HC1,  gradually 
at  first ;  then  add  8  cc.  of  ethyl  alcohol,  allow  the  salt  to  settle, 
and  decant  the  solution  into  a  test-tube.  (See  Note  3.)  Add 
from  a  dropper  just  two  drops  of  a  solution  of  turmeric  in 
ethyl  alcohol,  and  let  the  mixture  stand  10  minutes.  (Orange 
or  red  color,  presence  of  BORATE.)  Compare  the  color  with  that 
of  standards.  (See  Note  2.) 

Notes.  —  i.  The  red  color  which  boric  acid  gives  to  turmeric  is  in 
high  degree  dependent  upon  the  concentrations  of  the  HC1,  the  alcohol, 
and  the  turmeric ;  and,  to  secure  delicacy  of  the  test  and  results  that 
are  comparable  in  different  cases,  the  directions  given  must  be  closely 
adhered  to ;  in  which  case  f  mg.  of  B02  in  the  solution  tested  can  be 
detected. 

2.  To  make  sure  of  the  presence  of  borate  when  the  color  is  slight, 
and  to  estimate  the  quantity  present  in  other  cases,  the  color  should  be 
compared  with  standards  made  by  mixing  8  cc.  of  ethyl  alcohol,  8  cc. 
of  12  n.  HC1,  and  two  drops  of  turmeric  solution  with  3  cc.  of  water 
(as  a  blank)  or  with  3  cc.  portions  of  solutions  containing  known  quan- 
tities of  borate  (for  example,  i  mg.  and  10  mg.  of  B02). 

3.  Chlorate,  nitrite,  and  chromate,  because  of  their  strong  oxidizing 
power,  affect  the  color  of  the  turmeric.    Iodide  may  also  be  decom- 
posed by  the  oxygen  of  the  air,  and  the  color  of  the  I2  liberated  may 
obscure  the  borate  test.     Therefore,  in  case  any  of  these  constituents 
has  been  found  present,  evaporate  3  cc.  of  the  Na2COs  solution  (P.  101) 
to  dryness,  add  gradually  2  cc.  of  12  n.  HC1,  and  evaporate  again  to 
dryness.    To  the  residue  add  i  cc.  of  3  n.  Na2CO3  solution  and  2  cc.  of 
water ;  heat  to  boiling ;  filter  if  there  is  a  residue ;  and  treat  the  solu- 
tion with  reagents  as  directed  in  the  Procedure.     The  evaporation  with 
HC1  serves  to  reduce  the  oxidizing  substances  and  expel  HI,  and  the 
addition  of  Na2CO3  precipitates  any  chromium  that  may  be  present. 
The  solution  is  not  ordinarily  evaporated  with  HC1,  when  the  absence 
of  conflicting  substances  makes  it  unnecessary,  since  there  is  consider- 
able loss  of  boric  acid  in  evaporating  acid  solutions.         -3? 

Procedure  116.  —  Detection  of  Ar senate  and  Ar senile.  —  In 
case  in  P.  44  arsenic  was  found  present,  dilute  5  cc.  of  the  Na2C03 
solution  (P.  101)  with  10  cc.  of  water,  and  add  HNOs,  i  cc.  at  a 
time,  till  the  mixture  reddens  litmus  paper.  Then  add  NEUOH, 
a  few  drops  at  a  time,  till  the  mixture  turns  litmus  paper  blue, 


P.  116       DETECTION  OF  ARSENATE  AND  ARSENITE          161 

avoiding  an  excess.  Filter  if  there  is  a  precipitate.  Add  10  cc. 
of  Mg(NO3)2  reagent.  Let  the  mixture  stand  for  10  minutes, 
shaking  it  frequently.  (White  precipitate,  presence  of  ARSENATE 
or  PHOSPHATE.)  Filter,  and  wash  the  precipitate  with  i  n. 
NH4OH. 

To  the  nitrate  add  HC1,  i  cc.  at  a  time,  till  it  reddens  litmus 
paper,  and  pass  H2S  into  the  cold  solution  for  about  a  minute. 
(Immediate  yellow  precipitate,  presence  of  ARSENITE.)  (See 
Note  3.) 

Pour  on  to  the  filter  containing  the  Mg(N03)2  precipitate  i  cc. 
of  AgNO3  solution  to  which  a  few  drops  of  HAc  have  been  added. 
(Dark-red  residue,  presence  of  ARSENATE.)  In  case  the  residue 
is  yellow,  pour  repeatedly  through  the  filter  containing  it  a 
5  cc.  portion  of  HC1,  add  to  the  solution  i  cc.  of  KI  solution 
and  i  cc.  of  CCLj,  and  shake  the  mixture.  (Purple  color  in  the 
carbon-tetrachloride  layer,  presence  of  ARSENATE.) 

Notes.  —  i.  This  method  of  distinguishing  arsenate  and  arsenite 
depends  on  the  fact  that  arsenate  is  precipitated  by  Mg(NO3)2  reagent 
while  arsenite  is  not.  (See  Notes  2  and  3,  P.  44.)  To  prevent  precipita- 
tion of  magnesium  arsenite,  Mg(As03)2,  however,  the  NH4OH  concen- 
tration must,  as  directed  in  this  Procedure,  be  made  as  small  as  is 
consistent  with  securing  complete  precipitation  of  the  arsenate.  This 
precaution  was  not  necessary  in  P.  44,  where  the  arsenic  is  all  in  the 
form  of  arsenate  and  where  the  addition  of  a  large  quantity  of  NH4OH 
serves  to  produce  more  rapid  precipitation. 

2.  The  characteristic  dark-red  color  of  the  AgaAsCX  produced  by 
treatment  of  the  MgNH^sCX  with  AgNO3  solution  (see  Note  4,  P.  44) 
is  a  sufficient  confirmation  of  the  presence  of  arsenate  when  phosphate 
is  not  present.  Phosphate,  however,  is  also  precipitated  by  the 
Mg(N03)2  reagent;  and  it  is  converted  by  the  AgNO3  into  bright- 
yellow  AgsPCX  Moreover,  in  case  a  very  large  quantity  of  arsenite 
is  present,  it  may  be  partially  precipitated  by  the  Mg(N08)2  reagent ; 
and  it  will  then  be  converted  into  yellow  AgsAsOs  by  AgN03.  These 
yellow  precipitates  may  obscure  the  color  of  a  relatively  small  pro- 
portion of  arsenate;  and  in  this  case  the  further  confirmatory  test 
with  HC1  and  KI  becomes  necessary.  In  this  test  the  production  of  a 
purple  color  shows  the  presence  of  arsenate ;  for  I2  is  not  liberated  from 
iodide  by  either  phosphate  or  arsenite. 


162          DETECTION  OF  ARSENATE  AND  ARSENITE      P.  116 

3.  The  immediate  formation  of  a  yellow  precipitate  in  the  nitrate 
from  the  Mg(NO3)2  precipitate  is  a  conclusive  test  for  arsenite,  except 
in  case  antimony  is  present  in  the  substance.    This  element  may  pass 
into  the  Na2C03  solution,  and  it  then  would  yield  a  precipitate  with 
H2S,  which  might  be  mistaken  for  As2S3.     Moreover,  the  arsenite  test 
is  sometimes  obscured   by  other  elements,  especially  copper,  which 
may  pass  into  the  Na2CO3  solution  and  produce  dark  precipitates  with 
H2S.    Hence,  hi  case  antimony  is  present,  or  in  case  the  H2S  precipitate 
is  not  of  the  characteristic  yellow  color,  it  should  be  treated  by  P.  44, 
to  determine  whether  arsenic  is  present  in  it. 

4.  Even  jf  through  faulty  procedure  a  small  quantity  of  arsenate 
passes  into  the  nitrate  from  the  Mg(NO3)2  precipitate,  it  will  not  yield 
an  immediate  precipitate  with  H2S  in  the  cold  weakly  acid  solution 
(see  Note  7,  P.  21),  and  will  therefore  not  lead  to  a  mistaken  conclusion 
as  to  the  presence  of  arsenite. 


P.  117      DETECTION  OF  CARBONATE  AND  SULFIDE 


163 


DETECTION   OF   CARBONATE   AND  SULFIDE  IN   THE   ORIGINAL 
SUBSTANCE 

Procedure  117.  —  Detection  of  Carbonate  and  Sulfide  by  Dis- 
tillation. —  Set  up  in  the  way  shown  in  the  figure  an  appara- 
tus consisting  of  a  5o-cc.  round-bottom  hard-glass  flask  fitted 
with  a  rubber  stopper,  through  which  pass  a  delivery-tube  and 
a  safety-tube,  20-30  cm.  long,  leading  to  the  bottom  of  the 
flask.  Hold  the  flask  with  a  ring  or  clamp  in  an  inclined  posi- 
tion. Lead  the  end  of  the  delivery-tube  through  a  two-hole 
stopper  into  25  cc.  of  nearly  saturated  Ba(OH)2  solution  con- 
tained in  a  5o-cc.  flask  supported  in  a  beaker  of  cold  water. 

Place  0.5  g.  of  the  very  finely  powdered  substance  and 
about  0.2  cc.  of  granulated  Zn  in  the  distilling  flask.  (See  Note 
2.)  Boil  in  a  small  flask  for  about  a  minute  a  mixture  of  5  cc. 


of  water  and  5  cc.  of  HC1,  and  pour  it  into  the  distilling  flask 
with  the  aid  of  a  small  funnel  connected  temporarily  with  the 
safety-tube  by  means  of  rubber  tubing.  Heat  the  mixture 
slightly  at  first,  then  to  boiling,  and  boil  it  gently  till  2-3  cc. 
of  liquid  have  distilled  over. 

To  the  distillate  add  HAc,  i  cc.  at  a  time,  till  it  reddens  blue 
litmus  paper.     (White  precipitate  dissolving  partly  or  com- 


1 64          DETECTION  OF  CARBONATE  AND  SULFIDE       P.  117 

pletely  on  addition  of  the  acid,  presence  of  CARBONATE.)  Add 
to  the  mixture  5  cc.  of  PbAc2  solution.  (Black  precipitate, 
presence  of  SULFIDE.) 

In  case  in  P.  104,  no,  or  in  oxidizing  constituents  or  thio- 
cyanate  or  sulfite  were  found  present,  transfer  the  residue  un- 
dissolved  by  Na2CO3  solution  (P.  101),  with  the  filter-paper  if 
necessary,  to  a  5o-cc.  round-bottom  flask,  and  treat  it  as  de- 
scribed in  the  first  two  paragraphs  of  this  Procedure,  in  order 
to  detect  sulfide. 

Notes.  —  i.  On  heating  with  HC1  all  carbonates  are  decomposed 
with  evolution  of  CO2.  In  order  that  the  test  for  carbonate  may  be 
reliable,  care  must  be  taken  to  exclude  the  CO2  of  the  air  by  boiling  the 
acid  in  advance,  by  heating  the  mixture  regularly  so  that  no  air  sucks 
in  through  the  safety-tube,  and  by  keeping  the  Ba(OH)2  solution  away 
from  the  air  so  far  as  possible.  Even  with  these  precautions  it  is 
seldom  possible  to  prevent  the  absorption  of  enough  CO2  to  produce  a 
slight  turbidity.  Care  must  also  be  taken  to  prevent  any  of  the  dis- 
tilling liquid  from  being  thrown  over  mechanically  into  the  receiving 
flask,  since  many  of  the  non-volatile  constituents,  like  sulfate  and 
phosphate,  yield  precipitates  with  Ba(OH)2  solution. 

2.  In  case  in  P.  in  sulfite  has  been  found  present,  the  Zn  should 
not  be  added,  the  liquid  poured  into  the  flask  should  consist  of  5  cc.  of 
3%  H2O2  solution  and  5  cc.  of  HC1,  and  the  test  for  sulfide  in  the  dis- 
tillate should  be  omitted.    For,  in  the  presence  of  sulfite  the  test  for 
sulfide  is  unreliable  (see  Note  5),  and  the  test  for  carbonate  would  be 
obscured  by  the  evolution  of  SO2,  which  forms  with  Ba(OH)2  a  white 
precipitate  insoluble  in  HAc.    This  last  difficulty  is  removed  by  the 
addition  of  the  H2O2,  which  oxidizes  the  sulfite  to  sulfate  and  prevents 
any  SO2  from  passing  over. 

3.  Many  sulfides  are  decomposed  by  HC1  with  evolution  of  H2S; 
but  the  sulfides  of  the  copper  and  tin  groups,  and  certain  persulfides 
like  pyrite,  FeS2,  are  not  much  acted  upon  by  this  acid  alone.    These 
are,  however,  decomposed,  either  completely  or  to  a  large  extent,  when 
Zn  is  also  present,  in  virtue  of  its  reducing  action.    The  sulfides  of 
arsenic  are  only  slightly  acted  on  by  HC1  and  Zn ;   but,  as  they  dis- 
solve in  Na2CO3  solution,  sulfide  will  be  detected  in  P.  106  when  they 
are  present.    As  to  the  need  of  supplementing  the  test  for  sulfide  made 
in  P.  1 06  by  one  made  with  the  original  substance  or  with  the  residue 
insoluble  in  Na2CO3  solution,  see  Note  5,  P.  101. 


P.  117      DETECTION  OF  CARBONATE  AND  SULFIDE  165 

4.  In  case  arsenic  or  antimony  is  present  in  the  substance,  arsine 
(AsH3)  or  stibine  (SbH3)  may  be  evolved  upon  heating  the  substance 
with  Zn  and  HC1.    As  these  gases  are  extremely  poisonous  and  are  not 
absorbed  by  the  Ba(OH)z  solution,  the  distillation  should  be  carried  out 
under  a  hood  in  case  arsenic  or  antimony  is  present. 

5.  This  test  for  sulfide  may  fail  when  chlorate,  chromate,  nitrate, 
nitrite,  or  sulfite  is  present  with  it,  owing  to  destruction  of  the  H2S 
by  these  substances.     It  may,  moreover,  lead  to  the  conclusion  that 
sulfide  is  present  when  it  is  not,  in  case  the  substance  contains  sulfite 
or  thiocyanate,  since  these  constituents  also  yield  H2S  with  Zn  and 
HC1.    Hence,  in  case  any  of  these  conflicting  constituents  has  been 
found  present,  a  further  test  for  sulfide  is  made  upon  the  residue  undis- 
solved  by  Na2CO3  solution,  as  described  in  the  last  paragraph  of  this 
Procedure.    This  residue  does  not  contain  the  conflicting  constituents ; 
for  these  pass  completely  into  the  Na2C03  solution. 


ANALYSIS  OF  NATURAL   SUBSTANCES  AND   IGNEOUS 
PRODUCTS 


TABLE  XXI.  —  DETECTION  OF  SULFATE,  CARBONATE,  SULFIDE,  AND 
CYANIDE. 

Boil  0.5  g.  of  the  substance  with  HCl  and  Zn,  collecting  the  distillate  in 
Ba(OET)2  solution;  filter  the  mixture  left  in  the  distilling  flask  (P.  121). 


Filtrate  from  mixture 
in  distilling  flask. 
Add  BaCl2. 

Distillate. 
Precipitate  :  BaCO3.     (Shows  CARBONATE.) 
Solution  :  BaS,  Ba(CN)2. 

Precipitate:  BaSO4. 

(ShoWS  SULFATE.) 

To  a  part  of  the  mixture 
add  HAc  and  PbAc*. 

To  the  rest  of  the  mixture 
add  FeCk,  boil,  add  HCl. 

Black  precipitate  :  PbS. 
(Shows  SULFIDE.) 

Blue  precipitate  : 
Fe4(FeCN6)3. 

(Shows  CYANIDE.) 

Procedure  121.  —  Detection  of  Sulfate,  Carbonate,  Sulfide,  and 
Cyanide.  —  Treat  0.5  g.  of  the  very  finely  powdered  substance 
as  described  in  the  first  two  paragraphs  of  P.  117. 

Pour  the  contents  of  the  distilling  flask  on  to  a  filter.  Reject 
the  residue.  (See  Note  i.)  To  the  filtrate  add  5  cc.  of  BaCl2 
solution.  (White  precipitate,  presence  of  SULFATE.) 

In  case  the  substance  is  of  natural  origin,  treat  the  whole 
distillate  as  described  in  the  third  paragraph  of  P.  117  (to 
detect  CARBONATE  and  SULFIDE). 

In  case  the  substance  is  an  igneous  product,  treat  two-thirds 
of  the  distillate  as  described  in  the  third  paragraph  of  P.  117 
(to  detect  CARBONATE  and  SULFIDE)  ;  and  to  the  remaining  third 
add  i  cc.  of  FeCl2  solution,  boil  the  mixture  for  a  minute  or 
two,  and  add  HCl,  i  cc.  at  a  time,  till  the  solution  becomes 
acid.  (Blue  precipitate,  presence  of  CYANIDE.) 

Notes.  —  i.   All  sulfates  except  those  of  barium,  strontium,  and 
lead  are  very  soluble  or  moderately  soluble  in  dilute  HCl ;  and  that  of 
lead  is  decomposed  by  Zn.     Hence,  only  in  case  barium  or  strontium 
166 


P.  122 


NATURAL  AND  IGNEOUS  SUBSTANCES 


167 


has  been  found  present  and  the  substance  is  not  completely  dissolved 
by  dilute  acid  (as  used  in  P.  2)  is  it  necessary  to  test  a  natural  substance 
or  an  igneous  industrial  product  further  for  sulfate.  This  may  be 
done  by  transferring  from  the  filter  to  a  casserole  the  residue  from  the 
treatment  with  Zn  and  HC1,  boiling  it  for  5-10  minutes  with  10  cc. 
of  3  n.  Na,CO3  solution,  filtering,  acidifying  the  nitrate  with  HC1,  and 
adding  BaCU  solution. 

2.  As  to  the  precautions  to  be  observed  in  order  to  make  the  test 
for  carbonate  reliable,  see  Note  i,  P.  117. 

3.  As  to  the  action  of  HC1  and  Zn  on  sulfides  and  on  compounds  of 
arsenic  and  antimony,  see  Notes  3  and  4,  P.  117.     Constituents  that 
interfere  with  the  test  for  sulfide  (see  Note  5,  P.  117)  are  not  present  in 
natural  substances  and  igneous  industrial  products. 

4.  Cyanide  is  never  present  in  natural  substances,  but  is  occasionally 
present  in  igneous  products.     The  test  for  it  is  based  upon  the  formation 
of  Ba,Fe(CN)6  by  the  action  of  Ba(CN)2  on  the  Fe(OH)2  and  upon  the 
reaction  which  takes  place  upon  acidification  between  this  ferrocyanide 
and  the  ferric  salt  which  has  been  produced  by  the  oxygen  of  the  air. 
As  a  result  of  these  two  reactions,  ferric  ferrocyanide  (Prussian  blue) 
is  formed,  which  is  only  slightly  soluble  in  dilute  HC1. 


TABLE  XXII.  —  DETECTION  OF  CHLORIDE,  FLUORIDE,  AND  BORATE. 


Distil  i  g.  of  the  substance,  first  (A)  with  H2SO4  alone, 

then  (B)  -with  addition  of  CH&H  (P.  122). 


A.  First  distillate. 

B.  Second  distillate  : 
B(OCH3),. 
Add  HCl,  C2HbOH, 
and  turmeric. 

To  a  portion  add 
AgNOi. 

To  the  remainder  add 
NaAc  and  CaCl2. 

Precipitate:  AgCl. 

(Shows  CHLORIDE.) 

Precipitate  :  CaF2. 

(Shows  FLUORIDE.) 

Confirm  by  P.  112. 

Orange  or  red  color. 

(Shows  BORATE.) 

Procedure  122.  —  Detection  of  Chloride,  Fluoride,  and  Borate. 
—  Place  i  g.  of  the  very  finely  powdered  substance  in  a  SQ-CC- 
round-bottom  hard-glass  flask.  Pour  into  the  flask  6  cc.  of 
1 8  n.  H2SO4.  Insert  a  stopper  carrying  a  safety- tube  and 


1 68  NATURAL  AND  IGNEOUS  SUBSTANCES  P.  122 

delivery- tube,  arranging  the  apparatus  as  described  in  P.  117 
and  as  shown  in  the  figure  in  that  Procedure.  Lead  the  end 
of  the  delivery-tube  into  a  receiving  flask  containing  5  cc.  of 
water,  supported  in  a  beaker  of  cold  water.  Distil  the  mixture 
till  the  acid  becomes  oily  and  the  flask  becomes  filled  with  white 
fumes,  removing  the  flame  momentarily  once  or  twice  during 
the  distillation  to  cause  the  liquid  in  the  safety-tube  to  run 
down  into  the  flask. 

Boil  the  distillate  for  a  minute  or  two,  and  filter  it  if  it  is  tur- 
bid. To  one-fourth  of  the  solution  add  2  cc.  of  HN03  and 
1-3  cc.  of  AgN03  solution.  (White  precipitate,  presence  of 
CHLORIDE.)  To  the  remainder  of  the  solution  add  5  cc.  of  3  n. 
NaAc  solution  and  5  cc.  of  CaCl2  solution,  heat  the  mixture 
nearly  to  boiling,  and  let  it  stand  15  minutes.  (White  precipi- 
tate, presence  of  FLUORIDE.)  Treat  the  precipitate  by  P.  112, 
to  confirm  the  presence  of  fluoride. 

After  the  distilling  flask  has  cooled  completely,  pour  into  it 
gradually  8  cc.  of  pure  methyl  alcohol  (CHaOH),  and  mix  the 
liquids  by  shaking.  Distil  off  most  of  the  alcohol  into  a  re- 
ceiving flask  containing  a  mixture  of  3  cc.  of  water  and  8  cc.  of 
12  n.  HC1,  heating  the  sides  of  the  distilling  flask  with  a  small 
flame  to  prevent  bumping.  Pour  the  distillate  into  a  graduate, 
add  enough  ethyl  alcohol  to  make  the  volume  20  cc.,  and  then 
add  from  a  dropper  two  drops  of  a  solution  of  turmeric  in 
ethyl  alcohol.  (Orange  or  red  coloration,  presence  of  BORATE.) 
Compare  the  color  with  that  of  standard  solutions  containing 
known  quantities  of  borate. 

Notes.  —  i.  The  heating  with  concentrated  H2S04  liberates  from 
almost  all  substances,  except  certain  silicates,  the  HC1,  HF,  and  HBO2 
corresponding  to  any  chloride,  fluoride,  or  borate  present.  The  dis- 
tillation must  be  continued  until  the  acid  fumes  freely,  so  as  to  secure 
as  strong  a  decomposing  action  as  possible,  so  as  to  drive  over  into  the 
distillate  all  of  the  liberated  HF,  and  so  as  to  leave  the  acid  anhydrous 
for  the  subsequent  borate  test. 

2.  The  distillate  may,  in  addition  to  HC1  and  HF,  contain  H2S, 
HCN,  H2C03,  H2SiO3  (passing  over  as  SiFO,  and  S,  all  coming  from 
the  substance,  as  well  as  HzSO*  and  H2SO3  arising  from  volatilization 


P.  123  NATURAL  AND  IGNEOUS  SUBSTANCES  169 

or  reduction  of  the  H2S04  added.  Any  H2S,  HCN,  or  H2SO,  present 
is  removed  by  boiling  the  solution  before  making  the  tests  for  chloride 
and  fluoride,  since  otherwise  these  acids  would  produce  precipitates  in 
these  tests.  Any  H2Si03  or  S  present  is  filtered  out.  H2SO<  will  not 
pass  over  (unless  the  distillation  is  continued  much  too  long)  in  quantity 
sufficient  to  yield  a  precipitate  of  CaSO<,  especially  as  this  substance 
is  much  more  soluble  in  the  NaAc  solution  than  it  is  in  pure  water. 

3.  As  to  the  conditions  for  securing  precipitation  of  a  small  quantity 
of  fluoride  see  Note  i,  P.  103.     As  to  the  confirmatory  test,  see  the 
Notes  on  P.  112. 

4.  In  the  presence  of  concentrated  H2SO<  methyl  alcohol  reacts 
with  boric  acid  to  form  methyl  borate,  B(OCH3)3,  which  is  a  very  vola- 
tile liquid.    This  is  largely  decomposed  in  the  acid  distillate  with 
formation  of  boric  acid.    In  regard  to  the  color  test  for  borate  and  the 
comparison  with  standard  solutions,  see  Notes  i  and  2,  P.  115. 

Procedure  123.  —  Detection  of  Sulfate,  Fluoride,  Borate,  and 
Silicate  in  Substances  Not  Decomposed  by  Acids.  —  In  case  the 
residue  from  the  treatment  of  the  substance  with  HNO3  and 
HC1  by  P.  2  and  3  has  been  fused  with  Na2C03  by  P.  7,  treat 
one-half  of  the  aqueous  extract  obtained  in  P.  7  as  follows : 

Evaporate  two-thirds  of  the  solution  to  a  volume  of  about 
6  cc.,  and  treat  it  by  P.  111-112,  to  detect  sulfate  and  flouride. 

Make  one-third  of  the  solution  slightly  acid  with  HC1,  evapo- 
rate it  to  dryness,  moisten  the  residue  with  HC1,  evaporate  again 
to  dryness,  and  heat  the  residue  at  100-130°  till  it  is  perfectly 
dry,  keeping  the  casserole  in  motion  over  a  small  flame.  After 
cooling  add  just  6  cc.  of  6  n.  HC1  and  warm  the  mixture.  (Fine 
white  residue,  presence  of  SILICATE.)  Filter.  Treat  the  residue 
as  described  in  the  first  two  paragraphs  of  P.  5,  to  confirm  the 
presence  of  silicate.  To  the  solution  add  just  5  cc.  of  12  n.  HC1, 
8  cc.  of  CizHsOH,  and  two  drops  of  a  solution  of  turmeric 
in  C2H6OH.  Decant  the  solution  from  any  salt  that  has  sepa- 
rated into  a  test-tube,  and  let  it  stand  10  minutes.  (Orange 
or  red  color,  presence  of  BORATE.) 

Notes.  —  i.  This  Procedure  serves  to  detect  fluoride  and  borate 
in  certain  silicates  and  other  natural  or  ignited  substances  which 
are  not  acted  upon  by  hot  concentrated  HjSO*,  as  used  in  P.  122. 
Also  in  the  case  of  substances  not  completely  dissolved  by  the  treatment 


170  NATURAL  AND  IGNEOUS  SUBSTANCES          P.  1SS 

with  HN03  and  HC1  in  P.  2  and  3,  it  provides,  in  case  the  residue  has 
not  been  treated  with  HF  in  P.  5,  for  the  detection  of  silicate ;  or,  in 
case  the  residue  has  been  so  treated,  it  affords  a  more  reliable  and 
quantitative  detection  of  that  constituent.  In  such  difficultly  decom- 
posable substances  it  also  makes  more  certain  the  detection  of  sulfate. 

2.  This  method  of  testing  for  these  acidic  constituents  is  prescribed 
in  the  Procedure  only  in  the  cases  where  the  residue  from  the  treatment 
with  HN03  and  HC1  has  been  fused  with  Na2CO3  in  accordance  with 
the  directions  in  P.  6  and  7.    These  directions  require  such  fusion  either 
when  the  residue  could  not  conveniently  be  treated  with  HF,  or  when 
it  has  been  so  treated  and  is  not  completely  decomposed  by  it.    Yet, 
even  when  the  substance  is  completely  decomposed  by  the  treatment 
with   HF,  it  is  possible  that  it  may  not  be  sufficiently  acted  on  by 
hot  concentrated  H2SO4  to  liberate  the  HF  and  HB02  from  any  fluo- 
ride and  borate  present.    This  case  is  hardly  common  enough  to  warrant 
making  provision  for  it  a  part  of  the  regular  system  of  analysis ;  but 
to  make  absolutely  certain  that  these  constituents  are  not  missed,  the 
analyst  must,  whenever  the  substance  leaves  a  residue  after  the  treat- 
ment with  HNO3  and  HC1,  fuse  that  residue  with  Na2C03,  and  treat 
the  aqueous  extract  as  described  in  this  Procedure. 

3.  As  to  the  detection  of  sulfate  and  fluoride  see  the  Notes  on 
P.  in  and  112;   as  to  that  of  silicate,  see  Notes  6  and  7,  P.  3,  and 
Note  4,  P.  5 ;  as  to  that  of  borate,  see  Notes  i  and  2,  P.  115. 


APPENDIX 


I.  PREPARATION  OF  THE  REAGENTS. 


SOLUTIONS   OF  ACIDS 

Acetic,  6  n. :  Mix  350  cc.  of  99.5%  acid  with  650  cc.  of  water. 
Hydrochloric,  12  n. :  Use  the  c.  p.  acid  of  commerce  of  s.  g.  1.19. 
Hydrochloric,  6  n. :  Mix  12  n.  HC1  with  an  equal  volume  of  water. 
Hydrofluoric,  48% :  Use  the  pure  acid  sold  in  ceresin  bottles. 
Nitric,  16  n. :  Use  the  c.  p.  acid  of  commerce  of  s.  g.  1.42. 
Nitric,  6  n. :  Mix  380  cc.  of  HNO3  (s.  g.,  1.42)  with  620  cc.  of  water. 
Perchloric,  6  n. :  Mix  650  cc.  of  60%  c.  p.  acid  with  350  cc.  of  water. 
Sulfuric,  95%  :  Use  the  c.  p.  acid  of  commerce  of  s.  g.  1.84. 
Sulfuric,  18  n. :  Pour  465  cc.  of  95%  H2SO4  into  535  cc.  of  water. 
Sulfuric,  6  n. :  Pour  95%  H2S04  into  five  volumes  of  water. 

SOLUTIONS   OF   BASES 

Ammonium  hydroxide,  15  n. :  Use  the  c.  p.  product  of  s.  g.  0.90. 
Ammonium  hydroxide,  6  n. :   Mix  400  cc.  of  15  n.  NH4OH  with  600  cc.  of 

water. 
Barium  hydroxide,  0.4  n.  (approximately) :  Shake  60  g.  of  Ba(OH)2-  8H20 

with  1000  cc.  of  water  at  room  temperature,  and  decant  or  filter  the 

solution. 
Potassium  hydroxide,  6  n. :  Add  to  350  g.  of  best  c.  p.  KOH  enough  water 

to  make  the  volume  1000  cc. 
Sodium  hydroxide,  6  n. :    Add  to  250  g.  of  NaOH  "purified  by  alcohol" 

enough  water  to  make  the  volume  1000  cc. 

SOLUTIONS   OF  AMMONIUM   SALTS 

Acetate,  3  n. :  Dissolve  250  g.  of  the  solid  salt  in  enough  water  to  make  the 
volume  1000  cc. 

Carbonate,  9  n. :  Dissolve  250  g.  of  freshly  powdered  ammonium  carbonate 
in  enough  cold  6  n.  NH4OH  to  make  the  volume  1000  cc. 

Chloride,  3  n. :  Dissolve  160  g.  of  NH4C1  in  enough  water  to  make  the 
volume  1000  cc. 

Molybdate,  i  n.  in  (NH4)2MoO4,  3  n.  in  NH^Oa :  Dissolve  90  g.  of  the 
pure  ammonium  molybdate  of  commerce  (  (NH4)6Mo7O24-4H2O)  in 
100  cc.  of  6  n.  NH4OH,  add  240  g.  of  NH4NO»,  and  dilute  the  solu- 
tion to  looo  cc. 

171 


172 


PREPARATION  OF  THE  REAGENTS 


Sulfide:  Pass  H2S  gas  into  200  cc.  of  15  n.  NH4OH  in  a  bottle  immersed 
in  running  water  or  in  iced  water  till  the  gas  is  no  longer  absorbed ; 
then  add  200  cc.  of  15  n.  NH4OH  and  enough  water  to  make  the 
volume  1000  cc. 


SOLUTIONS   OF   OTHER  SALTS 

Dissolve  the  quantity  given  in  the  last  column  of  the  following  table  of 
each  of  the  salts  whose  formula  is  given  in  the  third  column  in  enough 
water  to  make  the  volume  1000  cc. 


Salt 

Normal 
Concert. 

Formula 

Formula 
Weight 

Grams  per 
Liter 

Barium  chloride     .     .     . 

i 

BaCl2-  2H2O 

244 

1  20 

Calcium  chloride*  .     .     . 

i 

CaCl2-6H20 

219 

no 

Cobalt  nitrate  .... 

0.3 

Co(N03)2-6H20 

291 

45 

Ferric  nitrate    .... 

i 

Fe(NO,),.9Hrf) 

404 

135 

Lead  acetate     .... 

i 

Pb(C2H302)2.3H20 

379 

190 

Lead  nitrate      .... 

i 

Pb(NOs)2 

33i 

165 

Mercuric  chloride  .     .     . 

0.2 

HgCl2 

271 

25 

Nickel  nitrate   .... 

I 

Ni(N03)2-6H20 

291 

145 

Potassium  chromate  .     . 

3 

K2Cr04 

194 

290 

Potassium  ferricyanide   . 

i 

K,Fe(CN)6 

329 

no 

Potassium  ferrocyanide  . 

i 

K4Fe(CN)6.3H20 

422 

105 

Potassium  iodide  .     .     . 

i 

KI 

166 

166 

Potassium  nitrite  .     .     . 

6 

KN02 

85 

500 

Potassium  oxalate      .     . 

3 

K2C204-H20 

184 

280 

Potassium  permanganate 

O.2 

KMnO4 

158 

32 

Potassium  thiocyanate   . 

I 

KSCN 

97 

IOO 

Silver  nitrate     .... 

I 

AgN03 

170 

170 

Sodium  acetate      .     .     . 

3 

NaC2H302-3H20 

136 

410 

Sodium  arsenite     .     .     . 

i 

NaAs02 

130 

130 

Sodium  carbonate  .     .     . 

3 

Na2CO, 

106 

1  60 

Sodium  nitrite!      .     .     . 

3 

NaNO2 

69 

210 

Sodium  phosphate      .     . 

i 

Na2HP04-i2H2O 

358 

120 

Sodium  sulfate  .... 

i 

Na2SO4-  loHzO 

322 

1  60 

Sodium  sulfite   .... 

i 

NajSOr  7H2O 

252 

125 

*  This  reagent  must  be  entirely  free  from  sulfate.  In  case  the  only  salt  available  con- 
tains sulfate,  mix  with  the  solution  i%  of  its  volume  of  BaClj  reagent,  let  the  mixture 
stand,  and  filter  it;  noting  on  the  label  that  the  reagent  contains  BaClj.  And  in  that 
case  prepare  a  separate  reagent  containing  no  BaClj,  for  use  in  P.  122. 

t  In  case  the  salt  contains  any  chloride,  mix  with  the  solution  i%  of  its  volume  of  AgNOj 
solution,  and  filter  the  mixture  after  vigorous  shaking.  Note  on  the  label  that  the  reagent 
contains  AgNOj. 


PREPARATION  OF  THE    REAGENTS  173 

SPECIAL  REAGENTS 

Bromine,  saturated  solution :  Shake  liquid  bromine  with  water,  leaving  a 
small  excess  of  it  in  contact  with  the  solution. 

Dimethylglyoxime,  o.i  n. :  Dissolve  12  g.  of  the  solid  in  1000  cc.  of  95% 
C2H5OH. 

Ferrous  chloride,  i  n. :  Dissolve  65  g.  of  FeCl2  in  enough  0.6  n.  HC1  to 
make  the  volume  1000  cc.,  and  keep  the  solution  in  contact  with 
iron  nails. 

Hydrogen  peroxide,  3  per  cent. 

Magnesium  ammonium  nitrate,  i  n.  in  Mg(N03)2,  3  n.  in  NE^NO*:  Dis- 
solve 130  g.  of  Mg(NO3)2  •  6  H20  and  240  g.  of  NH4NO,  in  water, 
add  35  cc.  of  6  n.  NH4OH,  and  dilute  to  1000  cc. 

Manganous  chloride  :  To  12  n.  HC1  add  powdered  MnCl2  •  4  H2O  until  after 
shaking  it  no  longer  dissolves. 

Potassium  mercuric  iodide,  0.5  n.  in  K2HgI4,  3  n.  in  NaOH :  Dissolve 
1 1 5  g.  of  HgI2  and  80  g.  of  KI  in  enough  water  to  make  the  volume 
500  cc. ;  add  500  cc.  of  6  n.  NaOH ;  and  decant  the  solution  from 
any  precipitate  that  may  form  on  standing.  Keep  this  stock  solu- 
tion in  the  dark. 

Potassium  pyroantimonate,  o.i  n.  in  K2H2Sb2O7,  0.5  n.  in  KOH:  Add 
22  g.  of  the  best  commercial  salt  to  1000  cc.  of  boiling  water,  boil 
for  a  minute  or  two  till  nearly  all  the  salt  is  dissolved,  quickly  cool 
the  solution,  add  35  cc.  of  6  n.  KOH  solution,  let  the  mixture  stand 
overnight,  and  filter  it. 

Sodium  cobaltinitrite,  0.3  n.  in  Na3Co(N02)6,  3  n.  in  NaN02,  i  n.  in  HAc: 
Dissolve  230  g.  of  NaNO2  in  500  cc.  of  water,  add  165  cc.  of  6  n.  HAc 
and  30  g.  of  Co(NO3)2  •  6  H2O,  let  the  mixture  stand  overnight,  filter 
or  decant  the  solution,  and  dilute  it  to  1000  cc. 

Sodium  sulfide,  3  n.  in  Na2S,  i  n.  in  Na2S2,  i  n.  in  NaOH :  Dissolve  480  g. 
of  Na2S .  9  H2O  and  40  g.  of  NaOH  in  water,  add  16  g.  of  sulfur,  shake 
the  mixture  till  the  sulfur  dissolves,  and  dilute  it  to  1000  cc. 

Stannous  chloride,  i  n.  in  SnCl2,"2  n.  in  HC1 :  Dissolve  115  g.  of  SnCl2  •  2  H2O 
in  170  cc.  of  12  n.  HC1,  dilute  the  solution  to  1000  cc.,  and  keep  it  in 
bottles  containing  granulated  tin. 

Thiourea :  Dissolve  100  g.  of  thiourea  in  1000  cc.  of  water. 

Turmeric:  Shake  an  excess  of  turmeric  powder  with  95%  C^jOH,  and 
filter  the  mixture. 


174  PREPARATION  OF  THE  REAGENTS 

SOLID  REAGENTS 

Aluminum  turnings.  Potassium  chloride. 

Antimony  powder.  Sodium  nitrate. 

Bismuth  dioxide  (sold  also  as  Silver  carbonate. 

sodium  bismuthate).  Sodium  carbonate  (anhydrous). 

Glass  beads.  Sodium  peroxide  (in  4  oz.  cans). 

Iron  powder.  Tin  (mossy). 

Potassium  chlorate  (powder).  Zinc  (finely  granulated). 
Quartz  powder. 

SOLVENTS 

Carbon  tetrachloride. 
Ethyl  alcohol  (95%). 
Ethyl  alcohol  (99%). 
Methyl  alcohol  (acetone-free). 


II.  PREPARATION  OF  THE  TEST-SOLUTIONS. 


Of  the  powdered  salt  whose  formula  is  given  in  the  middle  column  of  the 
following  table  weigh  out  the  number  of  grams  given  in  the  last  column, 
and  add  enough  hot  water  (or  acid  when  so  stated  in  the  footnote)  to  make 
the  volume  one  liter.  To  prepare  the  test-solutions,  which  contain  10  mg. 
of  the  constituent  per  cubic  centimeter,  dilute  these  stock  solutions,  which 
contain  100  mg.  of  the  constituent  per  cubic  centimeter,  with  nine  times 
their  volume  of  distilled  water.  In  a  few  cases  (indicated  by  the  letter  H) 
where  the  substance  is  not  sufficiently  soluble,  the  stock  solution  is  made 
up  so  as  to  contain  50  mg.  of  the  constituent  per  cubic  centimeter  and  must 
be  diluted  with  four  times  its  volume  of  water  to  yield  the  test-solution. — 
Since  these  solutions  serve  also  for  the  preparation  of  the  "unknown  solu- 
tions," the  purest  salts  that  can  be  purchased  should  be  employed. 


Constit- 

Formula 

Grams  per 

Constit-        Formula 

Grams  per 

uent 

of  Salt 

Liter 

uent 

of  Salt 

Liter 

Ag 

AgN03 

160 

Zn 

Zn(N03)2 

290 

Pb 

Pb(N03)2 

160 

Cr 

Cr(N03), 

460 

Hg(ous) 

Hg2(N03)2-2H20 

140  (a) 

Fe(ous)FeCl2 

230  (!) 

Bi 

Bi(N03)3-5H20 

230  (b) 

Fe(ic) 

Fe(N03)3.9H20 

715 

Cu 

Cu(N03)2-3H20 

380 

Mn 

Mn(NO3)2-6H20 

530 

Cd 

Cd(N03)2-4H2O 

275 

Ni 

Ni(N03)2-6H2O 

500 

Hg(ic) 

HgCl2 

yoH 

Co 

Co(NO3)2-6H2O 

500 

As(ous) 

As203 

13  to 

Ba 

BaCl2-2H20 

180 

As(ic) 

As205 

ISO 

Sr 

Sr(N03)2 

240 

Sb 

SbCl3 

190  (d) 

Ca 

Ca(N03)2-4H20 

590 

Sn(ous) 

SnCl2-  2H2O 

190  (e) 

Mg 

Mg(N03)2-6H20 

53oH 

Sn(ic) 

SnCU-3H2O 

270  (e) 

Na 

NaN03 

37° 

Al 

A1(N03)3-9H20 

7ooH 

K 

KN03 

260 

S 

Na2S-gH2O 

375H 

CrO4 

K2CrO4 

170 

CN 

NaCN 

190 

F 

KF 

305 

Fe(CN),IV 

K4Fe(CN)6.3H20  210 

C204 

K2C204-H20 

210 

Fe(CN)6m 

K3Fe(CN)6 

155 

N03 

NaN03 

140 

SCN 

KSCN 

170 

N02 

NaNO2 

ISO 

I 

KI 

130 

B02 

Na2B4Or  ioH2O 

oo 

Br 

KBr 

15° 

AsO4 

As206 

85 

Cl 

NaCl 

165 

As02 

Na^szOs 

ISO 

CIO, 

KC103 

75H 

P04 

Na2HPO4-i2H2O 

igoH 

S04 

Na2S04-ioH20 

340 

P04 

Ca3(P04)2 

160  (b) 

SO, 

Na2SOr7H2O 

3i5 

(a)  Dissolve  in  0.6  n.  HNOs.        (6)  Dissolve  in  3  n.  HNOs. 

(c)  Digest  with  500  cc.  of  1 2  n.  HC1 ;  then  add  500  cc.  of  water,  yielding  the  test-solution 

of  AsCls,  containing  10  mg.  of  As  per  cubic  centimeter. 

(d)  Dissolve  in  6  n.  HC1 ;  and,  in  making  the  test-solution,  dilute  with  2  n.  HC1. 

(e)  Dissolve  in  6  n.  HC1.      (/)   Dissolve  in  0.6  n.  HC1,  and  keep  in  contact  with  iron  nails. 


176  PREPARATION  OF  TEE  TEST-SOLUTIONS 

UNKNOWN   SOLUTIONS 

The  "unknown  solutions"  given  to  the  student  should  contain  the  con- 
stituents to  be  tested  for  in  quantities  which  are  definitely  known  by  the 
instructor.  As  a  rule  they  may  well  contain  in  10  cc.  300  mg.  of  one  of 
the  constituents,  30  mg.  of  another  of  the  constituents,  and  3  mg.  of  each 
of  two  or  three  of  the  remaining  constituents  of  the  group  in  question. 
Such  solutions  may  be  conveniently  prepared  in  advance  by  mixing  in  a 
250  cc.  bottle  60  cc.  of  the  stock  solution  of  the  first  constituent  (or  120  cc. 
if  it  is  half  -strength  as  shown  by  an  H  in  the  table),  6  cc.  of  the  stock  solu- 
tion of  the  second  constituent  (or  12  cc.  if  half  -strength),  and  6  cc.  of  the 
test-solutions  of  the  other  constituents,  and  diluting  with  enough  water 
to  make  the  volume  200  cc.  Of  these  "unknown  solutions"  just  10  cc. 
should  be  given  out  to  each  student  for  analysis.  When  time  permits  the 
analysis  of  two  unknown  solutions  in  any  group,  the  second  may  well  con- 
tain only  2  mg.  of  some  of  the  constituents. 

SOLID   TEST   SUBSTANCES 

Bleaching  powder.  FeS2  (pyrite). 

CaCOs  (powdered).  Fe3O4  (magnetite). 

Ca3(P04)2  (powdered).  NaC2H3O2. 
Cu(N03)2-3H20. 


Mixture  of  BiOCl  (30%),  Fe2(SO4),  (30%),  NaNO,  (30%),  Na2SOs-  7H2O 

(10%). 

Mixture  of  CaSO4-  2H2O  (60%),  CaCO3  (20%),  FeS2  (10%),  KCN  (10%). 
Mixture  of  CaF2  (5%),  Na^A-  ioH20  (5%),  NaCl  (5%),  sand  (85%). 


III.  APPARATUS  REQUIRED. 


Returnable 

2  Beakers,  lipped,  150  and  400  cc. 
2  Burners  (Tirrill). 
2  Casseroles,  30  cc. 
2  Casseroles,  75  cc. 
2  Casseroles,  150  cc. 
2  squares  Cobalt  glass, 
i  Crucible,  nickel,  30  cc. 
i  Filter-flask,  conical,  500  cc.,  with 
a  one-hole  rubber  stopper. 

1  Flask,  conical,  50  cc. 

4  Flasks,  conical,  100  cc.,  with  i 
two-hole  rubber  stopper. 

2  Flasks,  conical,  200  cc.,  with  i 

two-hole  rubber  stopper. 

1  Flask,  conical,  500  cc. 

2  Flasks,  round-bottom,  50  cc.,  with 

i  two-hole  rubber  stopper, 
i  Flask,  flat-bottom,  250  cc.,  with  a 

two-hole  rubber  stopper, 
i  Flask,     ring-neck,     flat-bottom, 

750  cc.,  with  a  two-hole  rubber 

stopper. 

1  Funnel,  50  mm. 

3  Funnels,  65  mm. 

Funnel  support  for  4  funnels. 
Graduate,  10  cc. 
Graduate,  50  cc. 
Key  for  desk. 

Mortar,  porcelain,  80  mm. 

Ring-stand  with  3  rings. 

Separating  funnel,  50  cc. 

6  Test-tubes,  100X12  mm. 

12  Test-tubes,  150X18  mm. 

2  Test-tubes,  hard  glass,   100X10 

mm. 

i  Test-tube  rack, 
i  Thistle-tube,  250  mm. 

1  Triangle,  nichrome,  50  mm. 

4  Watch  glasses,  40  mm. 

2  Watch  glases,  75  mm. 

2  Watch  glasses,  100  mm. 
i  Wing-top  for  burner. 


Not  Returnable 

ico  Filters,  7  cm.,  in  filter  box. 
loo  Filters,  9  cm.,  in  filter  box. 
12  Filters,  hardened,  5  cm. 
12  Filters,  hardened,  9  cm. 
75  cm.  Glass  rod,  5  mm.  diam. 
150  cm.  Glass  tubing,  6  mm.  outer 

diam. 
150  cm.  Glass  tubing,  7  mm.  outer 

diam. 

box  Labels. 

tube  Litmus  paper,  blue, 
tube  Litmus  paper,  red. 
box  Matches. 
Note-book. 
10  cm.  Platinum  wire. 
2  Rubber  nipples. 
150  cm.  Rubber  tubing,  bore  8  mm., 

wall  1.5  mm. 
75  cm.  Rubber  tubing,  bore  8  mm., 

wall  2  mm. 
30  cm.  Rubber  tubing,  pure,  bore 

5  mm. 
i  Sponge, 
i  Spoon,  horn,  bowl  i  cm.  long. 

1  Test-tube  brush. 

2  Towels. 

2  Wire  gauzes,  12X12  cm. 


IV.   SOLUBILITIES. 


GENERAL  STATEMENT 

Ammonium,  potassium,  and  sodium  salts :  all  very  soluble  in  water. 

Bismuth,  antimony,  tin,  and  mercury  salts :  all  hydrolyzed  by  water  with 
precipitation  of  the  hydroxide  or  basic  salt,  but  most  of  them  very 
soluble  in  i  n.  HC1  or  HNO3. 

Nitrates,  nitrites,  and  chlorates :  all  very  soluble  in  water  (except  AgN02). 

Carbonates,  sulfites,  borates,  oxalates,  phosphates,  arsenates,  and  arsenites : 
all,  except  those  of  the  alkali  elements,  slightly  soluble  in  water,  but 
very  soluble  in  i  n.  HC1  or  HNO3  (except  Sn3(PO4)4  and  Sn3(As04)4, 
which  are  very  slightly  dissolved  even  by  concentrated  HN03). 

Hydroxides:  all,  except  those  of  arsenic  and  the  alkali  elements,  very 
slightly  soluble  in  water  (except  also  those  of  barium,  strontium, 
and  calcium,  which  are  moderately  soluble  as  shown  in  the  following 
table) ;  but  all  very  soluble  in  i  n.  HC1  or  HN03  (except  those  of 
antimony  and  tin,  which  do  not  dissolve  in  HNO3). 

Chlorides,  bromides,  iodides,  thiocyanates,  and  sulfates:  all  very  soluble 
in  water,  except  as  shown  in  the  following  table  and  except  the 
mercurous  halides  and  HgI2. 

Cyanides,  ferro  and  ferricyanides :  very  slightly  soluble  in  water,  except 
those  of  the  alkali  and  alkaline-earth  group,  and  except  Hg(CN)2. 

Sulfides  of  the  silver-,  copper-,  and  tin-groups :  all  very  slightly  soluble  in 
water  and  in  cold  i  n.  HC1  or  HNOj. 

Sulfides  of  the  iron-group  and  of  zinc :  all  very  slightly  soluble  in  water, 
but  soluble  in  i  n.  HC1  or  HNO3  (except  FeS2,  NiS,  and  CoS). 

Sulfides  of  the  other  elements :  very  soluble  in  water,  or  decomposed  with 
separation  of  the  hydroxide  when  this  is  insoluble. 


178 


SOLUBILITIES 


179 


SALTS   OF   THE  ALKALINE-EARTH   AND   SILVER  GROUPS 

The  numbers  in  the  table  show  the  solubility  in  milli-equivalents  per  liter 
at  20°.  The  letters  v.s.  (very  soluble)  denote  a  greater  solubility  than 
i  normal.  In  the  case  of  the  carbonates  the  values  have  been  corrected  for 
hydrolysis  so  as  to  correspond  to  the  ion-concentration  product  in  the 
saturated  solution. 


Mg 

Ca 

Sr 

Ba 

Pb 

Ag 

Chloride     

v.s. 

v.s. 

v.s. 

v.s. 

70. 

O  OIOO 

Bromide     

v.s. 

v.s. 

v.s. 

v.s. 

45- 

o.ooo^ 

Iodide    

v.s. 

v.s. 

v.s. 

v.s. 

2.6 

O.OOOO2 

Thiocyanate   

v.s. 

v.s. 

v.s. 

V.S. 

28. 

0.0008 

Sulfide 

V  S 

V  S 

V  S 

V  S 

o  Oio4 

o  0147 

Sulfate 

v  s 

30 

I   e 

o  02 

o  28 

CQ 

Chromate 

v.s. 

60. 

12 

o  03 

o  0003 

o  16 

Carbonate  

20. 

O.2 

O.2 

O.2 

0.0004 

O.2 

Hydroxide  

0.3 

45- 

130. 

450. 

O.2 

0.18 

Fluoride 

2  8 

O  4. 

I  0 

18 

V  S 

Oxalate 

q 

o  oo 

O  ^ 

08 

O  OI2 

o  24 

V.   IONIZATION  VALUES. 


The  following  table  shows  approximately  the  percentage  of  the  substance 
which  is  dissociated  into  its  ions  in  o.i  normal  solution  at  25°.  In  the  case 
of  the  dibasic  and  tribasic  acids  the  value  opposite  the  formula  of  the  acid 
shows  the  percentage  of  the  first  hydrogen  that  is  dissociated,  and  that 
opposite  the  acid  ion  (HA~)  shows  the  percentage  of  it  dissociated  into 
H+  and  A=  for  the  case  that  these  two  ions  are  present  in  equal  quantities. 


Salts  of  type  B+A~  (e.g.  KN03) 84% 

Salts  of  type  B+2A=  or  B^A^  (e.g.  K2SO4  or  BaCl2) 73 

Salts  of  type  B+3A=  or  B+^A's  (e.g.  K3Fe(CN)6  or  A1C13)    ...          65 

Salts  of  type  B++A=  (e.g.  MgSO4) 40 

KOH.NaOH 90 

Ba(OH)2 80 

NH4OH i 

HC1,  HBr,  HI,  HSCN,  HN03,  HC103,  HC104 9° 

H2SO4,  H2Cr04,  H4Fe(CN)6,  H3Fe(CN)6 90 

H3PO4)  HjAsO^  H2S03,  H2C2O4,  HSO4~ 20-40 

HN02,  HF 7-9 

HAc,  HC2<V,  HSOr 1-2 

H2S,  H2CO3,  H2P(V,  HzAsOr,  HCrOr 0.1^x2 

HB02)  HAs02,  HCN,  HCO3-,  HC10 0.002-0.008 

HS-,  HP04=,  HAsO4= 0.0001-0.0002 

HOH 0.00,000,02 


i  So 


VI.   SPECIFIC  REDUCTION-POTENTIALS. 


The  following  values  are  those  of  the  actual  reduction-potentials  in  volts 
for  the  case  that  the  concentrations  of  the  ions  involved  are  i  molal.  They 
are  referred  to  the  potential  of  hydrogen  gas  at  a  pressure  of  one  atmosphere 
against  hydrogen-ion  at  i  molal,  taken  as  zero. 

To  find  the  actual  potential  for  other  concentrations  add  to  these  values 
for  each  tenfold  decrease  in  the  concentration  of  any  ion  present  in  the 
oxidized  state  0.06,  0.03,  or  0.02  respectively,  according  as  the  difference 
in  valence  in  the  two  states  is  unity,  two,  or  three,  respectively ;  and  sub- 
tract the  same  quantities  from  the  given  values  for  each  tenfold  decrease 
in  the  concentration  of  any  ion  present  in  the  reduced  state.  In  cases 
where  only  \  mol.  of  an  ion  or  other  dissolved  substance  (like  \  Hg2++  or 
\  C12)  is  present  in  the  oxidized  (or  reduced)  state,  add  (or  subtract)  only 
one-half  of  these  quantities  for  each  tenfold  decrease  in  its  concentration. 


Reduced 
State 

Oxidized 
State 

Reduction- 
Potential 

Reduced 
State 

Oxidized 
State 

Reduction- 
Potential 

K 

K+ 

2-93 

Bi 

Bi+++ 

-0.30 

Na 

Na+ 

2.72 

Cu 

Cu-1-1" 

-0.34 

Zn 

Zn++ 

0.76 

I- 

*I,  (solid) 

-0.53 

Fe 

Fe** 

0.43 

Fe++ 

Fe-*-"- 

-0.74 

Cd 

Cd-"- 

0.40 

Ag 

Ag+ 

-0.79 

Pb 

Pb++ 

0.13 

Hg 

i(Hft)~ 

-o.Sof 

Sn 

Sn++ 

O.IO 

i(H&)*+ 

Hg++ 

-0.92} 

*H2 

H+ 

0.00 

Br~ 

JBrj(i  m.) 

—  I.IO 

Sb 

Sb+-H- 

-O.I* 

ci- 

|Cl2(i  m.) 

-i-39 

Sn++ 

Sn^-1- 

—0.14* 

*  These  starred  values  denote  the  reduction-potentials  for  the  (unknown)  concentrations 
of  the  ions  which  prevail  in  solutions  i  formal  in  SbCU,  SnClj,  and  HjSnCl«  in  the  presence 
of  i  normal  HC1.  The  reduction-potentials  cannot  be  given  for  i -molal  concentration  of 
the  respective  ions,  since  the  degrees  of  ionization  and  complex-formation  in  antimony  and 
tin  chloride  solutions  are  imperfectly  known. 

f  The  values  of  the  actual  potentials  of  these  two  mercury  combinations  are  —0.27  and 
—0.63,  respectively,  in  the  presence  of  i  n.  Cl~  ion,  owing  to  the  very  small  solubility  of 
HgiCb  and  the  small  ionization  of  HgCU. 

181 


VH.  ATOMIC  WEIGHTS   OF  THE  COMMON  ELEMENTS. 


Aluminum Al 

Antimony Sb 

Arsenic As 

Barium Ba 

Bismuth Bi 

Boron B 

Bromine Br 

Cadmium Cd 

Calcium Ca 

Carbon C 

Chlorine Cl 

Chromium      .     .     .     .  Cr 

Cobalt Co 

Copper Cu 

Fluorine F 

Gold Au 

Hydrogen H 

Iodine    .  .  I 


27.1 

1  20.  2 

Iron      .     .     . 
Lead     .     .     . 

.    ,    .    Fe     55-84 
Pb  207  20 

74.96 

137-37 
208.0 
IO.Q 
70  02 

Magnesium    . 
Manganese    . 
Mercury    .     . 
Molybdenum 
Nickel 

.     .     .    Mg    24.32 
.     .     .    Mn    54.93 
.     .     .    Hg  200.6 
.     .     .     Mo    96.0 

Ni      58  68 

112.40 
4.O  O7 

Nitrogen   .     . 
Oxygen 

.     .     .     N      14.008 
O       1600 

12.005 
3546 
^2  O 

Phosphorus    . 
Potassium 
Silicon  . 

.     .     .    P       31-04 
.    .     .    K      39.10 
.     .    Si      28.3 

58.97 
63-57 
19.0 
197.2 
1.008 
126.92 

Silver    .     .     . 
Sodium 
Strontium 
Sulfur   .     .     . 
Tin  .     .     .     . 
Zinc      .     .     . 

.     .     .    Ag  107.88 
.     .     .    Na    23.00 
.     .     .    Sr      87.63 
.     .     .    S       32.06 
.     .     .     Sn   118.7 
.     .     .    Zn     65.37 

182 


Printed  in  the  United  States  of  America. 


CHEMICAL  LABORATORY  OF  HARVARD  COLLEGE 

THE  ALKALINE  EARTH  AND  ALKALI  GROUPS 

Evaporate  or  dilute  the  filtrate  or  solution  containing  the  above 
groups  to  fifty  cc.  Add  ammonia  until  strongly  alkaline,  heat  to 
boiling,  and  add  ammonium  carbonate  to  complete  precipitation. 
Filter,  reserving  the  filtrate;  wash  the  precipitate  twice  with  hot 
water,  then  dissolve  it  in  ten  to  twenty  cc.  of  hot  dilute  acetic 
acid,  pouring  through  several  times. 

To  one  cc.  of  the  acetic  acid  solution,  boiling  hot,  in  a  test  tube, 
add  potassium  chromate  until  the  solution  is  orange,  waiting  five 
minutes  if  necessary.  Yellow  precipitate  indicates  barium.  If 
barium  is  present,  heat  the  rest  of  the  acetic  acid  solution  to  boiling 
and  add  enough  potassium  chromate  to  turn  it  orange.  Filter 
repeatedly  through  the  same  paper  until  clear.  The  tip  of  the 
paper  may  be  torn  off,  wound  in  a  platinum  wire,  moistened  with 
concentrated  hydrochloric  acid  and  held  in  the  Bunsen  flame. 
Yellowish-green  flame  proves  barium. 

If  potassium  chromate  was  added  to  the  main  portion  of  the 
acetic  acid  solution,  add  ammonia  to  the  filtrate  from  the  barium 
chromate  until  alkaline,  ammonium  carbonate  in  excess,  filter, 
wash  and  dissolve  the  precipitate  in  acetic  acid  as  before;  otherwise 
continue  with  the  main  portion  of  the  acetic  acid  solution,  to  which 
no  potassium  chromate  was  added.  To  one  cc.  of  this  solution  in  a 
test  tube  add  an  equal  volume  of  calcium  sulphate,  waiting  half 
an  hour  and  rubbing  the  inside  of  the  tube  with  a  glass  rod  if  no 
precipitate  appears  at  once.  White  precipitate  proves  strontium. 
If  so,  heat  to  boiling,  and  add  dilute  sulphuric  acid  in  excess  to 
the  remainder  of  the  solution,  boiling  hot,  and  allow  to  stand  for 
half  an  hour;  filter  off  the  strontium  sulphate  together  with  any 
calcium  sulphate  precipitated  with  it. 

To  the  filtrate  add  ammonia  in  excess  and  ammonium  oxalate; 
if  no  precipitate  appears  at  first,  rub  the  inside  of  the  tube  with  a 
glass  rod  at  intervals  for  one  hour.  White  precipitate,  calcium 
oxalate,  proves  calcium. 

During  the  above  analysis  the  filtrate  from  the  carbonates  of 
barium,  strontium  and  calcium  is  boiled  down  to  a  bulk  of  twenty 
cc.  To  the  boiling  solution  add  a  few  drops  dilute  sulphuric  acid; 
white  precipitate  shows  barium  or  strontium  not  precipitated  by 
ammonium  carbonate.  Make  alkaline  with  ammonia,  and  add  a 
few  drops  of  ammonium  oxalate ;  white  precipitate  shows  calcium 
not  precipitated  by  ammonium  carbonate.  Filter.  If  no  precipi- 
tate was  obtained  originally  with  ammonium  carbonate,  these 
tests  must  none  the  less  be  carried  out,  the  precipitates  filtered  off 
separately,  moistened  with  concentrated  ammonium  chloride,  and 
identified  by  flame  tests. 

The  filtered  solution,  containing  the  chlorides  of  magnesium  and 
the  alka-li  group,  is  now  entirely  free  from  barium,  strontium,  and 


calcium.     To  two  cc.  add  di-sodium  pi      ii||il||B 
Rub  the  inside  of  the  tube  with  a  glass  i      ||(|tf 
hour  if  no  precipitate  appears.    White  pi          frf(     Q01  371  222       9 
crystalline  indicates  magnesium.     See  Procedure  »y.     evaporate 
the  rest  of  the  solution  to  dryness,  then  heat  the  bottom  and  sides 
of  the  dish  nearly  to  redness  to  expel  entirely  all  ammonium  salts; 
the  expulsion  is  probably  complete  when  no  more  fumes  can  be 
seen  on  removing  the  flame  and  blowing  over  the  dish.    After  cool- 
ing, take  up  the  whole  residue  with  water.     If  magnesium  was 
found  in  the  side  test,  it  must  now  be  removed  by  adding  barium 
hydroxide  in  excess  and  filtering  off  the  magnesium  hydroxide. 
Heat  the  filtrate  to  boiling,  add  dilute  sulphuric  acid  in  consider- 
able excess,  and  filter  off  the  barium  sulphate.    If  magnesium  was 
proved  absent  add  no  barium  hydroxide.    Five  cc.  dilute  sulphuric 
acid,  however,  must  be  added. 

Evaporate  the  filtrate  or  solution,  which  contains  only  sulphates 
of  potassium,  sodium,  and  lithium  until  no  more  fumes  of  sul- 
phuric acid  can  be  detected.  If  no  residue  can  be  seen,  the 
alkali  group  is  absent.  Cool  any  residue  and  add  a  few  drops  of 
water.  Dip  a  platinum  wire  into  this  concentrated  solution,  and 
examine  the  flame,  using  both  the  color  screen  and  the  spectro- 
scope. Sodium  must  not  be  reported  unless  the  yellow  flame  is 
both  brilliant  and  persistent.  Add  five  cc.  of  water  and  divide 
into  three  parts.  To  one  part  add  two  cc.  sodium  nitrite,  two  cc. 
acetic  acid,  boil  five  minutes.  Nesslerize  a  portion  and  if  no 
ammonium  is  present,  test. the  remainder  for  potassium  with 
sodium  cobaltinitrite.  Yellow  precipitate  proves  potassium  present 
if  ammonium  salts  have  been  completely  decomposed.  If  the 
solution  is  acid,  barely  neutralize  another  portion  with  very  dilute 
potassium  hydroxide,  and  test  *for  sodium  with  potassium 
pyroantimonate.  White  precipitate,  which  should  be  crystalline 
and  settle  quickly  after  shaking,  proves  sodium,  but  gelatinous 
precipitates,  which  usually  contain  alkaline  earth  metals,  must  be 
tested  for  sodium  in  the  flame.  To  the  third  portion  add  half  a 
cc.  of  sodium  hydroxide  and  two  cubic  centimeters  di-sodium 
phosphate.  Heat  to  boiling,  add  one  cc.  alcohol,  .shake  and  boil. 
White  precipitate  is  lithium  or  sodium  or  magnesium  phosphate. 
To  confirm  lithium,  filter,  wash  with  ammonia,  moisten  with 
concentrated  ammonium  chloride,  and  test  in  the  flame. 

Ammonium  must  be  detected  in  a  fresh  portion  of  the  original 
solution.  Add  to  five  cc.  in  a  small  beaker  five  cc.  concentrated 
sodium  hydroxide,  and  cover  with  a  watch  glass  to  whose  concave 
side  adheres  a  moistened  strip  of  red  litmus  or  yellow  turmeric 
paper.  Warm,  but  do  not  allow  to  spatter.  A  uniform  alkaline 
reaction  over  the  whole  strip  proves  ammonia. 

Read  pages  81-90  in  NOTES  and  study  all  the  notes  which  are 
pertinent  to  the  above  method. 

9-24-20—300 


